The best way to create a list that contains all prospects who complete the Contact Us form, but will later remove any prospects if they become marked as opted out, is to use a dynamic list. A dynamic list is a list that automatically adds or removes prospects based on criteria that you define. In this case, the criteria would be based on the form completion and the opt out status of the prospect. A dynamic list would also add prospects back to the list if they opt back in. The other options are not as efficient or effective as a dynamic list, as they would require manual intervention or additional automation steps

The automation tool that would best achieve this is to create a page action set to tag prospects as having viewed it and add them to a list. 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 behaviors2. 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 tags the prospect as having viewed it and adds them to a list of pricing page visitors.

Explanation
When working on a list email, the Marketing Manager has three options that are not available when working on an Account Engagement email template. They are:
* Ability to send the email immediately. This option allows the Marketing Manager to send the list email to the selected recipients as soon as the email is ready, without scheduling it for a later date or time. This
* option is useful for urgent or time-sensitive messages6
* Ability to schedule the email. This option allows the Marketing Manager to choose a specific date and time to send the list email to the selected recipients. This option is useful for planning ahead or aligning with the best time to reach the audience6
* Recipient and suppression lists. This option allows the Marketing Manager to select which lists of prospects will receive the list email, and which lists of prospects will be excluded from receiving the list email. This option is useful for targeting and segmenting the audience based on their attributes or behaviors6

The feature that the Marketing Cloud Account Engagement administrator should use to gather a prospect's company name and job title, but only once they have captured the prospect's first name, last name, and email address in a previous form submission, is progressive profiling. Progressive profiling is a feature that allows the administrator to display different fields on a form based on the information that the prospect has already provided. This way, the administrator can avoid asking the same questions repeatedly and collect more information gradually. Progressive profiling can be enabled on a form by selecting the option "Always display even if previously completed" for the fields that are required, such as first name, last name, and email address, and selecting the option "Only display if progressive profiling is enabled and the field is blank" for the fields that are optional, such as company name and job title. This way, the form will only show the optional fields once the required fields are filled out. Always display even if previously completed, reCaptcha, and dependent fields are not features that can achieve the same goal as progressive profiling. Always display even if previously completed is an option that can be used to enable progressive profiling, but it is not a feature by itself. reCaptcha is a feature that can be used to prevent spam submissions on a form, but it does not affect the fields that are displayed on the form. Dependent fields are fields that are displayed or hidden based on the value of another field, but they do not depend on whether the prospect has already provided the information or not. Reference Progressive Profiling

Explanation
The expected behavior if a prospect refreshes the page after initially submitting the form with the "Always Display Form After Submission" setting enabled is that the form would be displayed on the page once again (B). This setting allows the form to be submitted multiple times by the same prospect, which is useful for fields that are set to always be displayed, such as reporting issues or comments. The prospect would not receive an error message (A), be redirected to a thank-you page, or see the thank-you content (D), as these options are not compatible with the "Always Display Form After Submission" setting. References: Account Engagement Form Troubleshooting and FAQ