Which document describes the monitoring activities required for each of the three lines of defence?

Comprehensive and Detailed Explanation From Exact Extract:

The three lines of defence model in MSP ensures that risks and performance are managed effectively through independent layers of oversight. The Assurance Approach sets out how each of these lines of defence will monitor, report, and intervene when necessary.

The MSP Foundation, 5th Edition states:

“The assurance approach defines how the three lines of defence will be used in the programme and describes how assurance will be provided across all areas.”

Thus, option D is correct.