The report you need is available in the SharePoint admin center under Reports > Data access governance. This feature provides snapshot and activity reports that detail permissions, sharing links, and potential oversharing across both SharePoint sites and OneDrive account.
Reference:
https://learn.microsoft.com/en-us/sharepoint/data-access-governance-site-permissions-report?

Explanation:

The correct answer is adding a public DNS record . Microsoft documents that when you add a custom domain to Microsoft 365, you must first prove ownership of that domain before Microsoft 365 can use it for services such as Exchange Online, SharePoint, and user sign-ins. The standard verification method described by Microsoft is to add a DNS record at your domain registrar or DNS hosting provider. Microsoft commonly uses a TXT record for verification, although in some cases an MX record can also be used depending on the setup flow. This is why "adding a public DNS record" is the right completion for the sentence.
The other choices are not the standard Microsoft 365 domain verification process. Microsoft's admin guidance does not use confirming your business address , uploading a certificate , or uploading a webpage as the normal method for proving ownership of a domain in Microsoft 365. Domain verification is specifically tied to DNS because DNS is the authoritative public system used to prove control over the domain name.

To capture a user's activity and check for data exfiltration using Microsoft Purview Insider Risk Management, follow these steps:
1. Prerequisites and Licensing
2. Initial Setup
*-> 3.Create an Insider Risk Policy
Navigate to Solutions > Insider Risk Management > Policies in the Microsoft Purview portal.
Select Create policy and choose a template focused on exfiltration, such as Data leaks or Data theft by departing users.
Scope the policy: Under the Users and groups page, select Include specific users and groups and add the individual user you want to monitor.
Select Indicators: Choose specific exfiltration indicators like downloading files from SharePoint, sending emails with attachments to external recipients, or copying data to USB devices.
Triggering Events: Define what starts the monitoring, such as a Data Loss Prevention (DLP) policy match or a specific exfiltration activity.
4. Monitor and Investigate
Reference:
https://learn.microsoft.com/en-us/purview/insider-risk-management-configure