Explanation
The scenario that would convert an anonymous visitor into an identified prospect is submitting a Web2Lead form that is linked to Account Engagement via a form handler. This is because a form handler captures the visitor's information and passes it to Account Engagement, where a new prospect record is created. Creating a Lead in Salesforce which syncs to Account Engagement as a new prospect record (A) will not convert an anonymous visitor, as it does not involve any interaction with the visitor. Scanning the badge of someone who visited a booth at a recent event (B) will not convert an anonymous visitor, as it does not involve any online activity that can be tracked by Account Engagement. Opening an email sent by a sales rep using the Send Account Engagement Email functionality in Salesforce (D) will not convert an anonymous visitor, as it requires the visitor to already have a prospect record in Account Engagement. References: Anonymous Visitors

An automation rule can match a prospect more than once if you enable Repeat Rule. Otherwise, an automation rule will only affect a prospect one time.
Explanation:
An automation rule is a criteria-based action that is triggered when a prospect matches the rule. By default, an automation rule will only affect a prospect one time, unless you enable the Repeat Rule option. If you enable the Repeat Rule option, the automation rule can match a prospect more than once, as long as they meet the criteria each time. Automation rules do not run every time, since they are evaluated based on a schedule that you can set. Reference: [Automation Rules], [Repeat Automation Rules]

The baseline grade assigned to a prospect is D. A Marketing Cloud Account Engagement grade is a rating based on how well a prospect's profile matches your ideal customer profile. The prospect rating is expressed as a letter grade that ranges from F (worst) to A+ (best). Marketing Cloud Account Engagement assigns an initial grade of D to all prospects. You can then adjust the grade up or down based on the prospect's attributes, such as industry, job title, company size, location, etc. The grade reflects how well the prospect fits your target market and how qualified they are for your product or service. You can create different grading profiles for different personas or segments, and assign prospects to profiles based on certain criteria. For more details -> 345

Two methods that would accomplish providing access to an eBook once a form has been submitted are using a form completion action to send an autoresponder email with the eBook, and re-directing the prospect to the eBook instead of showing the form's Thank You Content. A form completion action is a way of defining what happens after a prospect submits a form, such as sending an email, adding to a list, or adjusting a score.
An autoresponder email is a type of email that is automatically sent to a prospect when they perform a specific action, such as submitting a form. A Thank You Content is a web page that is displayed after a prospect submits a form, such as a confirmation message or a link to a resource. Using a form completion action to initiate an auto-download of the eBook, or including a link to the eBook in the Below Form section of the form are not methods that would accomplish providing access to an eBook once a form has been submitted, as they either involve downloading the eBook before submitting the form, or displaying the link to the eBook before submitting the form4. References: 4: [Create a Form]

Explanation
The custom email links generated by Marketing Cloud Account Engagement are tracked for link clicks, page visits and email opens. Link clicks are recorded when a prospect clicks on a link in an email. Page visits are recorded when a prospect visits a page on your website that has the Marketing Cloud Account Engagement tracking code. Email opens are recorded when a prospect opens an email and downloads the invisible image that Marketing Cloud Account Engagement inserts in the email. Form completions and unsubscribes are not tracked by the custom email links, but by other methods.