Explanation
A segmentation rule is a type of rule that allows marketers to segment their prospects based on specific criteria, such as field values, activities, or scores. A segmentation rule runs only once, as soon as the user completes the rule creation process. The rule will not run again, unless the user manually re-runs it or schedules it to run at a later date. A segmentation rule does not run continuously, up to five times, or as many times as the user specifies when they create the rule1. References: 1: Segmentation Rules

Explanation
You can send an automated email to a prospect after they fill out a form using either an automation rule or a completion action. An automation rule is a criteria-based action that is triggered when a prospect matches the rule. You can use an automation rule to send an email to prospects who fill out a specific form or any form. A completion action is a task that is performed after a prospect successfully completes a marketing element. You can use a completion action to send an email to prospects who fill out a particular form. You cannot use a segmentation rule to send an email, since segmentation rules are used to segment prospects into lists or update their field values. References: [Automation Rules], Completion Actions, [Segmentation Rules]

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

Explanation
According to the Salesforce documentation, the two Marketing Cloud Account Engagement form actions that would accomplish enabling bot protection on their forms are honeypot technique and reCaptcha. Bot protection is a feature that helps prevent automated bots from submitting forms and creating fake prospects.
Honeypot technique is a method that uses a hidden field in the form that is not visible to human users, but can be detected by bots. If the hidden field is filled out, the form submission is rejected as a bot submission.
reCaptcha is a service that uses a challenge-response test to verify that the form submitter is a human and not a bot. The user can enable either or both of these options in the form settings to add bot protection to their forms.
Enabling HTTPS, or Hypertext Transfer Protocol Secure, is a method that encrypts the communication between the user's browser and the web server, but it does not prevent bots from submitting forms. Dependent fields are fields that are displayed or hidden based on the value of another field, but they do not prevent bots from submitting forms either. References: Salesforce documentation

Explanation
Marketing Cloud Account Engagement writes to Lead and Contact records in Salesforce by default, as these are the primary objects that store prospect information. Opportunity, Account, and Case records are not written to by default, but can be synced using custom fields and connectors1. References: 1: Marketing Cloud vs.
Marketing Cloud Account Engagement