According to the Salesforce documentation, when a prospect is permanently deleted from Marketing Cloud Account Engagement, all the associated data and activities are also deleted and cannot be restored. However, if the prospect fills out a new Marketing Cloud Account Engagement form and submits their information, a new prospect record will be created with the same email address as the deleted one, but without any of the previous data and activities. The new prospect will have a new unique ID and will not be linked to the deleted one in any way. References: Salesforce documentation

The two characteristics that must be true of the list that the prospect can view and opt out of on the Marketing Cloud Account Engagement Email Preference Center are that it is a Static List and that it is a CRM Visible List. A Static List is a list that is manually populated by the marketer or the prospect. A prospect can opt out of a Static List by clicking on the unsubscribe link in an email or by visiting the Email Preference Center and deselecting the list. A CRM Visible List is a list that is synced with a Salesforce campaign and can be viewed and reported on in Salesforce. A prospect can be removed from a CRM Visible List if they no longer meet the criteria of the Salesforce campaign, such as having a certain score or status. Option B is not correct because a Dynamic List is a list that is automatically populated by Marketing Cloud Account Engagement based on criteria that match prospect fields. A prospect cannot opt out of a Dynamic List, but they can be removed from the list if they no longer meet the criteria. Option D is not correct because a Public List is a list that is visible on the Email Preference Center and can be opted in or out by the prospect. A Public List can be either a Static List or a Dynamic List, but it is not a characteristic that affects the prospect's removal from the list.
References: Marketing Cloud Account Engagement Email Preference Center Best Practices - Salesforce Ben, Create a Custom Email Preference Center Page - Salesforce, Crafting Clear Marketing Communication with Marketing Cloud Account Engagement Email Preference Centers

According to the Salesforce documentation, the user should resolve the issue of not being able to choose the specific email template when sending a new list email by editing the email template to make it available for "List Emails". An email template is a reusable design that defines the content and layout of an email. An email template can be configured to be available for different types of email sends, such as one-to-one emails, list emails, or autoresponders. A list email is an email that is sent to a specific list of prospects, such as a newsletter or a promotion. To use an email template for a list email, the user must edit the email template and check the box for "List Emails" in the email template settings. This will make the email template available to choose when sending a new list email. Editing the list to be available for "Email Sending", "Email Templates", or editing the email template and choosing the appropriate list are not the correct ways to resolve the issue of not being able to choose the specific email template when sending a new list email, as they are either irrelevant or incorrect options for the list or the email template settings. Reference: Salesforce documentation

You can delete a prospect through the prospect table actions. The prospect table is a list of all the prospects in your Marketing Cloud Account Engagement account, which you can access by navigating to Prospects > Prospect List. From there, you can select one or more prospects and click on the Tools button to see the available actions. One of the actions is Delete, which will permanently remove the selected prospects from your Marketing Cloud Account Engagement account. You can also delete a single prospect by clicking on their name and then clicking on the Delete button on their record.
Answer A is incorrect because you cannot delete a prospect through a completion action. A completion action is a task that is performed after a prospect successfully completes a marketing element, such as a form, landing page, or custom redirect. You can use completion actions to perform various actions on prospects, such as sending emails, adjusting scores, changing campaigns, or adding tags, but not deleting them. Answer B is incorrect because you cannot delete a prospect through an automation rule. An automation rule is a criteria-based action that is triggered when a prospect matches the rule. You can use automation rules to perform various actions on prospects, such as sending emails, adjusting scores, changing campaigns, or adding tags, but not deleting them. Answer C is incorrect because you can delete prospects, as explained above. Reference: Prospect List, [Delete Prospects], [Completion Actions], [Automation Rules]

The best way to configure a thank you email to be sent after the form on the "Request a Demo" landing page is submitted is to configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted. An autoresponder email is an email that is automatically sent to a prospect after they complete a desired activity, such as submitting a form or clicking a custom link. You can use autoresponder emails to thank your prospects, confirm their actions, or provide them with additional information or content. A completion action is an action that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity, such as submitting a form or clicking a custom link. You can use completion actions to perform actions on the prospects who complete the activity, such as adding them to a list, assigning them to a user, or changing their field values. To configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted, you need to go to Marketing > Forms > Forms > Request a Demo > Completion Actions > Add New Action > Send Autoresponder Email, and select the email that you want to send.