Explanation
Mailable Database usage is the number of prospects in an account that are opted in and can receive emails. It does not include prospects that are opted out, in the recycle bin, or have a hard bounce or do not email status.
Therefore, the Mailable Database usage for the marketer with 4,000 total prospects, 3,000 of which are opted in, 500 of which are opted out, and 500 of which are in the recycle bin, is 3,000 (A). The other options (B, C, D) are incorrect because they either include prospects that are not mailable or exclude prospects that are mailable. References: Marketing Cloud Account Engagement - Salesforce.com

The process to add a prospect who visits a pricing page to a list is to create a page action. A page action is a feature that allows you to track and act on the behavior of prospects who visit specific pages on your website, such as your pricing page, product page, or thank you page. You can use page actions to perform actions on the prospects who visit the page, such as adding them to a list, assigning them to a user, or changing their field values. You can also use page actions to customize the content or layout of the page based on the prospect's attributes or behaviors3. To create a page action, you need to specify the URL of the page that you want to track, and the actions that you want to execute when a prospect visits the page. For example, you can create a page action that matches the URL of your pricing page, and adds the prospect to a list of pricing page visitors.

If a LenoxSoft marketer selects the option "Redirect the prospect instead of showing the form's Thank You Content" when creating a new Account Engagement landing page, the expected behavior when a prospect submits a form designed to show Thank you Content is that the prospect will be redirected to another webpage . This option overrides the form's Thank you Content and sends the prospect to the specified URL instead. The prospect will not continue to see the form upon submission (A), nor will the discrepancy between the two assets cause an error (B). The form's Thank you Content will not display (D), as it is replaced by the redirect option. Reference: Redirect Account Engagement Forms to a Success Page

According to the Salesforce documentation, the explanation for why the completion actions did not occur when the client submitted their Marketing Cloud Account Engagement form to test the form's completion actions is: A) The client has a visitor filter set up to filter activities from their IP address. Completion actions do not occur on filtered visitors. A visitor filter is a feature that allows users to exclude certain visitors from being tracked and reported in Marketing Cloud Account Engagement. A visitor filter can be based on IP address, location, or domain. A visitor filter can help users to avoid tracking their own activities or those of their competitors. A completion action is an automation tool that can be used to perform an action after a prospect successfully completes a marketing element, such as submitting a form, clicking a link, or visiting a web page. A completion action can be used to perform actions such as adding a prospect to a list, assigning a prospect to a user, sending an autoresponder email, or adjusting a prospect's score. However, completion actions do not occur on filtered visitors, as they are not tracked or reported in Marketing Cloud Account Engagement. Therefore, if the client has a visitor filter set up to filter activities from their IP address, the completion actions will not occur when they submit the form. The 'email' form field being set up to exclude free email addresses, the client not un-pausing the completion actions, or kiosk mode being enabled on the form are not the explanations for why the completion actions did not occur when the client submitted their Marketing Cloud Account Engagement form to test the form's completion actions, as they are either irrelevant, incorrect, or nonexistent options for the form or the completion actions. Reference: Salesforce documentation

According to the Salesforce documentation, the option that is not a way to assign a custom role to a user is: D) Add a user to a Custom Role directly from their User Record. A custom role is a feature that allows users to create and assign custom permissions and access levels to other users in Marketing Cloud Account Engagement. A custom role can be created and managed by the administrator in the Admin tab in Marketing Cloud Account Engagement, and it can be assigned to new or existing users. A custom role can be assigned to a user by using different methods, such as:
Edit an existing Default Role, and it will automatically convert to a Custom Role: A default role is a predefined role that comes with Marketing Cloud Account Engagement, and that has a fixed set of permissions and access levels. A default role can be edited by the administrator in the Admin tab in Marketing Cloud Account Engagement, and it will automatically convert to a custom role when the changes are saved. The administrator can then assign the custom role to a user or a group of users.
Assign new users to a previously created Custom Roles during the import process: A new user is a user that is not yet added to Marketing Cloud Account Engagement, and that needs to be created and assigned a role. A new user can be imported by the administrator in the Admin tab in Marketing Cloud Account Engagement, and they can be assigned to a previously created custom role during the import process. The administrator can use a CSV file to import multiple new users at once, and to assign them to different custom roles.
Add a batch of existing users to a Custom Role using table actions: An existing user is a user that is already added to Marketing Cloud Account Engagement, and that has a role assigned to them. An existing user can be added to a custom role by the administrator in the Admin tab in Marketing Cloud Account Engagement, and they can use the table actions to add a batch of existing users to a custom role at once. The administrator can select multiple existing users from the User List, and then click on the Change Role option in the table actions. The administrator can then choose the custom role that they want to assign to the selected users.
The option that is not a way to assign a custom role to a user is: Add a user to a Custom Role directly from their User Record. A user record is a feature that shows the detailed information and settings of a user in Marketing Cloud Account Engagement. A user record can be accessed from the Admin tab in Marketing Cloud Account Engagement, and it can show different sections of information for the user, such as details, permissions, or activity. The user record can also show the role that is assigned to the user, but it cannot be used to add a user to a custom role directly. The user record can only be used to edit the user's information, such as name, email, password, or timezone, but not the user's role. To add a user to a custom role, the administrator needs to use one of the methods mentioned above, such as editing an existing default role, assigning new users during the import process, or adding a batch of existing users using table actions. Reference: Salesforce documentation