When tracking access to files hosted outside of Pardot, the best feature to use is a Custom redirect. This tool allows Pardot users to create trackable URLs which can then be used to direct prospects to external files. The interactions with these URLs are tracked, capturing data whenever a prospect accesses the link. This enables marketers to gather analytics on file access even when the content itself is not hosted within Pardot, providing valuable insights into prospect engagement with external resources.

Explanation
The event that would make the Salesforce connector become unverified is the connector user's Salesforce password was changed. A Salesforce connector is a feature that allows you to connect your Marketing Cloud Account Engagement account with your Salesforce CRM system, and enable bi-directional data sync and alignment between sales and marketing. To verify the Salesforce connector, you need to have a connector user, which is a Salesforce user that Marketing Cloud Account Engagement uses to access your Salesforce data and perform actions on your behalf. The connector user needs to have the appropriate permissions and settings in Salesforce, such as the API Enabled and Modify All Data permissions, and the security token2. If the connector user's Salesforce password was changed, the security token would also change, and Marketing Cloud Account Engagement would not be able to authenticate with Salesforce. This would cause the Salesforce connector to become unverified, and the data sync between Marketing Cloud Account Engagement and Salesforce would stop. To fix this, you need to update the connector user's password and security token in Marketing Cloud Account Engagement, and re-verify the Salesforce connector

The automation tool that should be used to accomplish this is creating an automation rule to register prospects based on the form submission. An automation rule is a rule that runs continuously in the background and matches prospects based on the criteria you set. You can use an automation rule to perform actions on the matched prospects, such as registering them for a webinar. For example, you can create an automation rule that matches prospects who have submitted a specific form, and then use the action Register for Webinar to register them for the webinar that is integrated with Marketing Cloud Account Engagement.

When an engagement studio program is created with an Email Send action step immediately followed by an Email Open trigger step with a 3 day wait, it means that the program will wait for up to 3 days for the prospect to open the email. If the prospect opens the email within the 3 day period, they will immediately progress down the "Yes" path (A). The prospect will not wait for any additional time after opening the email (B), nor will they progress down the "No" path (C, D). Reference: Engagement Studio Overview

Explanation
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.
References: Prospect List, [Delete Prospects], [Completion Actions], [Automation Rules]