Microsoft Purview Data Loss Prevention (DLP) is the feature designed to prevent the unintentional or accidental sharing of sensitive information. An administrator can create a DLP policy that identifies content containing specific sensitive information types, such as Social Security numbers. The policy can then be configured to automatically block emails containing this information from being sent to external recipients, display a policy tip to the user, and notify an administrator. This directly prevents the type of incident described in the scenario.

B. Activity Explorer is a tool for viewing and filtering audit logs of user activities. It is used for investigation after an event, not for prevention.

C. Retention labels are used for data lifecycle management, determining how long data should be kept or when it should be deleted, not for preventing data exfiltration.

D. eDiscovery (Premium) is a tool used to find and collect data for legal cases and investigations. It is a reactive tool, not a preventative control.

1. Microsoft. (2024). Learn about data loss prevention. Microsoft Learn. Retrieved from https://learn.microsoft.com/en-us/purview/dlp-learn-about-dlp. (The introductory paragraph states, "you can create and manage DLP policies... to help prevent the unintentional or accidental sharing of sensitive information.")

2. Microsoft. (2024). Create and Deploy data loss prevention policies. Microsoft Learn. Retrieved from https://learn.microsoft.com/en-us/purview/dlp-create-deploy-policy. (This guide details how to configure policies to "protect content in Exchange Online email" and block messages containing sensitive information.)