What is necessary to ascertain the availability of the Commanding Officer (CO) and Executive Officer (XO)?

To ascertain the availability of the Commanding Officer (CO) and Executive Officer (XO), checking flagged signals on board is the appropriate action. Flagged signals often provide critical information regarding the status and location of officers, including their availability for duty. These signals communicate various messages and intentions that directly reflect the officers' current status or any instructions they may have issued.

In contrast, while the other options provide valuable information in different contexts, they do not specifically pertain to the current availability of the CO and XO. For instance, consulting the compass report primarily pertains to navigation, reviewing embarkation lists focuses on personnel assigned to a specific mission or deployment, and examining the crew roster can provide insight into crew assignments but may not reflect the real-time status or location of the CO and XO. Therefore, checking flagged signals is the most direct and effective approach for identifying the availability of these key personnel.