Explanation
By default, if the information in Salesforce differs from the information in Marketing Cloud Account Engagement, the information in Salesforce will override the information in Marketing Cloud Account Engagement. This means that Salesforce is the master source of truth for most fields, and any changes made in Salesforce will overwrite the values in Marketing Cloud Account Engagement. However, this behavior can be customized for each field in Marketing Cloud Account Engagement, by choosing one of the following options:
Use Marketing Cloud Account Engagement's value, Use Salesforce's value, or Use the most recently updated record. You can also prevent a field from syncing at all by choosing Do not sync. For more details -> 45

The following activities would make a prospect active in Marketing Cloud Account Engagement:
* Submitting a Marketing Cloud Account Engagement form, which indicates the prospect's interest and consent to receive communications from the company.
* Clicking on a custom redirect, which tracks the prospect's engagement with external links or downloadable content.
* Visiting a Marketing Cloud Account Engagement-tracked page, which captures the prospect's browsing behavior and page views on the company's website. References: [Marketing Cloud Account Engagement Prospect Activities]

Forms are web forms that capture prospect information and allow you to perform actions based on the form submission. You can set a form to Always Display, which means that the form will always show when someone returns to the page, even if they have already filled out the form before. This option is useful if you want to allow prospects to update their information or submit the form multiple times. Setting a form to Always Display does not affect the form conversion rate, prevent spammers from filling out the form, or ensure the prospect gets scored and graded. Those factors depend on other settings and criteria. Reference: Forms and Form Handlers, Form Display Options

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

According to the Salesforce documentation, the actions that can be done with the gear icon on the prospect list are: B) Assign, C) Delete, and D) Edit. The gear icon is a feature that allows users to perform different actions on a prospect or a group of prospects from the Prospect List. The gear icon can be accessed from the Prospects tab in Marketing Cloud Account Engagement, and it can show different options for the selected prospect or prospects, such as assign, delete, or edit. The assign option allows the user to assign the prospect or prospects to a user, a group, or a queue. The delete option allows the user to delete the prospect or prospects from Marketing Cloud Account Engagement. The edit option allows the user to edit the prospect or prospects' information, such as name, email, or custom fields. The copy option is not an action that can be done with the gear icon on the prospect list, as it is not an available option for the prospect or prospects.
References: Salesforce documentation