According to the Salesforce documentation, the two capabilities that are true for completion actions are: C) Completion actions cannot be applied directly on a landing page, and D) Completion actions do not execute on image file downloads. A completion action is an automation tool that can be used to perform an action after a prospect successfully completes a marketing element, such as clicking a link in an email, submitting a form, or visiting a web page. A completion action can be used to perform actions such as adding a prospect to a list, assigning a prospect to a user, sending an autoresponder email, or adjusting a prospect's score. Completion actions cannot be applied directly on a landing page, as landing pages do not have completion actions. However, completion actions can be applied on the forms or custom redirects that are embedded on the landing page. Completion actions do not execute on image file downloads, as image file downloads are not considered as completion events. However, completion actions can be executed on other types of file downloads, such as PDFs or Word documents. Completion actions are not applied retroactively, as they are only applied to prospects who complete the marketing element after the completion action is created. Completion actions do not apply to both visitors and prospects, as they only apply to prospects who have been identified by their email address or their Marketing Cloud Account Engagement tracking cookie. Reference: Salesforce documentation

Explanation
The best place to look on a prospect record to help determine the reason a prospect is unmailable is the Lifecycle and Profile section. This section shows the mailability status of the prospect, which can be one of the following: Mailable, Mailable - Transactional Emails Only, Undeliverable, Unmailable, or Unsubscribed4. The mailability status is determined by various factors, such as the prospect's email opt-out preference, the do not email flag, the hard bounce or soft bounce detection, and the prospect's location in the recycle bin5. The Lifecycle and Profile section also shows the email address, the email preference center, and the email opt-out date of the prospect, which can provide more information about the prospect's mailability

Automation rules and segmentation rules are both types of Marketing Cloud Account Engagement automation that allow you to perform actions based on criteria. However, the main difference between them is that automation rules run continuously and segmentation rules run once. Automation rules are always on and listening for changes or activity, and they can apply actions to prospects that match the criteria now or in the future. Segmentation rules are designed to pull a one-time list of prospects, then apply the set actions, such as adding them to a list or a Salesforce campaign. Once run, segmentation rules have to be copied to run again

Explanation
https://static1.squarespace.com/static/54359ec1e4b0d2eefcc96bb0/t/5b930f624ae2373f8cb06bad/153636438862

The information that is required when creating a prospect manually is campaign, email, profile, and score. A prospect is a potential customer who has expressed some interest in your products or services, and whose information you have captured in Marketing Cloud Account Engagement. You can create a prospect manually in Marketing Cloud Account Engagement by entering their information in a form, or by importing them from a file. When creating a prospect manually, you need to provide the following information:
* Campaign: The marketing initiative that you use to track the first touch point with the prospect, such as a trade show, a webinar, or a Google Ad. You can select a campaign from the drop-down menu, or use the default campaign that you set in your account settings.
* Email: The email address of the prospect, which is used as the unique identifier for the prospect record.
You need to enter a valid email address for the prospect, or the prospect will not be created.
* Profile: The category that you use to segment your prospects based on their characteristics, such as industry, role, or product interest. You can select a profile from the drop-down menu, or use the default profile that you set in your account settings.
* Score: The numerical value that indicates the level of interest or engagement of the prospect in your products or services. You can enter a score for the prospect, or use the default score of 0 that is assigned to new prospects.
Other information that you can provide when creating a prospect manually are:
* First Name: The first name of the prospect, which can be used for personalization or segmentation.
* Last Name: The last name of the prospect, which can be used for personalization or segmentation.
* Company: The name of the company that the prospect works for, which can be used for reporting or segmentation.
* Website: The URL of the website that the prospect visits or owns, which can be used for tracking or segmentation.
* Assign To: The user that you want to assign the prospect to, who will be responsible for following up with the prospect. You can select a user from the drop-down menu, or use the default user that you set in your account settings.