Comprehensive and Detailed in Depth Explanation:

In the SAP S/4HANA Migration Cockpit, the data retention period determines how long migration data (e.g., staging tables or files) is retained before being archived or deleted, aligning with data governance and compliance requirements.

Option A (When the project status is "Not started"): Correct. The retention period is defined during the initial setup of a migration project in the Migration Cockpit, before any data is loaded or processed. This is part of the project configuration in the "Create Project" step, ensuring clarity on data lifecycle management from the outset.

Option B (When the project status is "Finished"): Incorrect. By the "Finished" stage, migration activities are complete, and the retention period is already in effect—changes are no longer allowed.

Option C (When the project status is "In progress"): Incorrect. While adjustments can be made during "In progress," the retention period is a foundational setting established earlier.

Option D (When the project status is "Completed"): Incorrect. "Completed" is not a standard status in the Migration Cockpit; "Finished" is the final status, and retention is predefined.