The only required field for a record in Salesforce to be synced or created as a prospect record in Marketing Cloud Account Engagement is the email address. Marketing Cloud Account Engagement uses the email address as the unique identifier for prospects, so it is mandatory for any record to sync between the two platforms. Other fields, such as name, company, or phone, are optional and can be mapped between Marketing Cloud Account Engagement and Salesforce, but they are not required for the sync to happen. For more details -> 123

A form handler is a feature that allows you to use your own forms to post your data to Marketing Cloud Account Engagement. A form handler is similar to a form, but instead of displaying the form on your website, you point your form to Marketing Cloud Account Engagement. This allows you to submit data to Marketing Cloud Account Engagement as well as your own database3. Form handlers are useful when you want to keep the look and feel of your existing forms, or when you want to capture data from multiple forms using one form handler

A marketing user is a type of user role in Marketing Cloud Account Engagement that has access to various features and functions related to marketing operations. A marketing user can see users and groups, create new lists, forms, landing pages, and send list emails, manage prospects, and import prospects. A marketing user cannot access the administration settings, create or edit users, or sync with Salesforce.
Answer E is incorrect because none of the above is not a valid answer, as explained above. Reference: User Roles, Create and Edit Users

According to the Salesforce documentation, the two prospect activities that trigger a sync from Marketing Cloud Account Engagement to Salesforce are: A) Submitting a form, and B) Opening an email. A form submission is an activity that occurs when a prospect fills out and submits a Marketing Cloud Account Engagement form or a form handler. An email open is an activity that occurs when a prospect opens an email sent from Marketing Cloud Account Engagement. These two activities will trigger a sync from Marketing Cloud Account Engagement to Salesforce, meaning that the prospect's information and activity data will be updated in Salesforce. A sync from Marketing Cloud Account Engagement to Salesforce happens when a prospect is created or updated in Marketing Cloud Account Engagement, or when a prospect performs a significant activity in Marketing Cloud Account Engagement. Clicking a custom redirect or unsubscribing from email are not activities that trigger a sync from Marketing Cloud Account Engagement to Salesforce, as they are not considered significant activities in Marketing Cloud Account Engagement. A custom redirect is a trackable URL that can be used to measure and report on the clicks of any online content. An unsubscribe is an action that occurs when a prospect opts out of receiving emails from the sender. These two activities will not trigger a sync from Marketing Cloud Account Engagement to Salesforce, unless they are combined with other activities that do trigger a sync, such as submitting a form or opening an email. Reference: Salesforce documentation