Microsoft Purview Data Loss Prevention (DLP) provides controls to prevent Microsoft 365 Copilot from processing sensitive organizational content when generating responses.

Option D is correct because the Contoso technical requirement explicitly states that Microsoft 365 Copilot responses must not use content from project documents stored on SharePoint Site1.

Microsoft Purview supports a dedicated Microsoft 365 Copilot and Copilot Chat DLP policy location. Administrators can configure a DLP rule using the Content contains → Sensitivity labels condition and the Prevent Copilot from processing content action.

This prevents Copilot from processing files with the specified sensitivity labels or incorporating their contents into generated responses, even when users have SharePoint permissions.

For Contoso, the project documents can be identified using their distinctive project-code format, PR-123456789, and classified with an appropriate sensitivity label. The DLP policy can then exclude those labeled documents from Copilot processing.

Option A is incorrect as a complete solution because sensitivity labels and auto-labeling policies classify and protect documents but do not independently enforce the required Copilot processing restriction.

Option B is incorrect because Insider Risk Management detects potentially risky user activities.

Option C is incorrect because Information Barriers restrict communication and collaboration between designated user segments.

Technical consideration: The DLP policy requires an appropriate sensitivity label to be applied to the project documents. DLP rules for Copilot cannot directly exclude stored files merely by matching their project-code pattern.

Therefore, a DLP policy is the required enforcement mechanism.