According to the Salesforce documentation, a prospect's score can be changed through automation rules, scoring model changes, or completion actions. A prospect's score is a numerical value that indicates the level of interest and engagement of a prospect, based on their activities, such as opening an email, clicking a link, or submitting a form. A prospect's score can be changed by using different methods, such as:
Automation rules: An automation rule is an automation tool that can be used to update a prospect's score when the prospect meets certain criteria, such as prospect field values, activities, or scores. For example, an automation rule can be created to increase a prospect's score by 10 points when the prospect visits a specific web page.
Scoring model changes: A scoring model is a feature that allows users to customize the scoring rules for different types of activities, such as email opens, form submissions, or custom redirects. A scoring model can be changed by editing the default scoring model in Marketing Cloud Account Engagement, or by creating a custom scoring model for a specific group of prospects. For example, a scoring model can be changed to assign more points to an email open, or to create a different scoring model for prospects from a certain industry.
Completion actions: A completion action is an automation tool that can be used to adjust a prospect's score after the prospect successfully completes a marketing element, such as submitting a form, clicking a link, or visiting a web page. A completion action can be used to increase or decrease a prospect's score by a specific amount, or to reset a prospect's score to zero. For example, a completion action can be created to decrease a prospect's score by 5 points when the prospect unsubscribes from an email.

If a new Lead record is created in Salesforce without an email address and the Salesforce connector is set to "automatically create prospects in Marketing Cloud Account Engagement", no new prospect will be created in Marketing Cloud Account Engagement. This is because email address is a required field for creating a prospect in Marketing Cloud Account Engagement, and without it, the sync will fail. The Lead record will remain in Salesforce, but it will not have a corresponding prospect in Marketing Cloud Account Engagement1. Reference: Marketing Cloud Account Engagement Sync Behavior

If business hours are enabled for an engagement studio program, any email steps will be executed only during the specified hours. If a prospect enters an email step outside of the business hours, the email will be queued until the next business hour. Therefore, if a prospect enters a Send Email step at 4:30pm on Friday, and the business hours are Monday-Friday from 10am-4pm, the email will be sent on Monday at 10am

Explanation
When using completion actions for list emails, you should keep in mind that completion actions based on email link clicks only execute once per prospect, and completion actions based on email opens will retroactively apply if added after the email send. These are important considerations because they affect how your completion actions will work and what results you will see. For example, if you want to add a prospect to a list based on a link click, you should know that the completion action will only fire the first time the prospect clicks the link, not every time. Similarly, if you want to change a prospect's score based on an email open, you should know that the completion action will apply to all prospects who opened the email, even if they opened it before you added the completion action

Explanation
The three things that will affect the prospect records based on these activities are:
* Bob's prospect record will show him clicking on the link and filling out the form. This is because when an unknown visitor fills out a form, they become a known prospect and their previous activities are tracked and associated with their prospect record1.
* Kate's prospect record will show her clicking on the link and filling out the form. This is because Marketing Cloud Account Engagement tracks the activities of known prospects who interact with your marketing assets, such as custom redirects and forms2.
* Kate's prospect record will have a tag of "clicked" applied to it. This is because the custom redirect has a completion action to add a tag of "clicked" to all prospects that click it, and completion actions are actions that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity