The best way to be able to track the number of visitors who access a file on a web page that the company does not own is to use a custom redirect. A custom redirect is a Marketing Cloud Account Engagement feature that allows you to track any link on your website or a third-party website and measure the clicks and conversions.
You can create a custom redirect for the link to the file and use the custom redirect URL instead of the original link on your web page. This way, you can track how many visitors click on the link and access the file, and also see which prospects are interested in the file. A Marketing Cloud Account Engagement form, a Marketing Cloud Account Engagement tracking code, or a page action are not suitable for this purpose, as they are used for different scenarios. A Marketing Cloud Account Engagement form is used to collect visitor information and convert them to prospects. A Marketing Cloud Account Engagement tracking code is used to track the page views and activities of visitors and prospects on your own website. A page action is used to perform certain actions when a prospect views a specific page on your website. References Custom Redirects

Marketing Cloud Account Engagement syncs with Salesforce every 10 minutes. Marketing Cloud Account Engagement and Salesforce work together to keep data aligned between the two platforms. Some of the data that syncs bi-directionally are leads, contacts, and campaigns. Some of the data that syncs one-directionally from Salesforce to Marketing Cloud Account Engagement are accounts, opportunities, and tasks. Marketing Cloud Account Engagement syncs with Salesforce every 10 minutes by default, but you can also trigger a sync manually or by certain actions. For example, you can sync a prospect from their record in Marketing Cloud Account Engagement or Salesforce, or you can sync a group of prospects by using automation rules, segmentation rules, or imports. You can also sync a prospect by changing their Marketing Cloud Account Engagement campaign, assigned user, or email opt-out status. You can view the sync status and history on the prospect record in Marketing Cloud Account Engagement or Salesforce. For more details -> 18192021

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

Explanation
According to the Salesforce documentation, the marketer can accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce by using a dynamic list. A dynamic list is a list of prospects that is updated automatically based on certain criteria, such as prospect field values, activities, or scores. A dynamic list can be used to segment prospects based on their opportunity stage in Salesforce, and to add or remove prospects from the list when the opportunity stage changes. For example, a dynamic list can be created for prospects whose opportunity stage is Negotiations, and another dynamic list can be created for prospects whose opportunity stage is Closed Won.
When an opportunity moves from Negotiations to Closed Won in Salesforce, the prospect associated with that opportunity will be removed from the Negotiations list and added to the Closed Won list in Marketing Cloud Account Engagement. A completion action, a page action, or an automation rule are not the best tools to accomplish the goal of creating different Marketing Cloud Account Engagement lists to correspond with the different stages of the buying cycle and automatically updating the prospect list membership based on the opportunity stage change in Salesforce, as they are related to other aspects of automation, such as performing actions after a prospect completes a marketing element, visits a web page, or meets certain criteria, but not segmenting prospects based on their opportunity stage in Salesforce. References: Salesforce documentation

Explanation
Creating or marking an opportunity as lost will result in the change of a prospect's score. This is because opportunities are one of the factors that affect the scoring of prospects in Marketing Cloud Account Engagement. Scoring is a numerical value that indicates the level of interest or engagement of a prospect in your products or services. Scoring is based on the actions and activities of prospects, such as opening emails, clicking links, filling out forms, visiting landing pages, and creating or updating opportunities. When an opportunity is created or marked as lost, the prospect's score will change accordingly. For example, if you have a scoring rule that adds 50 points to a prospect's score when an opportunity is created, and subtracts 25 points when an opportunity is lost, then creating or marking an opportunity as lost will affect the prospect's score by those amounts.
Answer B is incorrect because creating or marking an opportunity as lost will not result in no change of a prospect's score, as explained above. References: Scoring, Opportunities