Which flag would be hoisted to indicate the absence of the Commanding Officer?

The correct choice for indicating the absence of the Commanding Officer is the Third Substitute flag. In naval flag signaling, the Third Substitute is specifically used for this purpose, signifying that the Commanding Officer is not present. The use of the Third Substitute effectively communicates to other vessels that the leadership on board may be temporarily lacking, reducing confusion and ensuring that proper procedures are maintained until the return of the Commanding Officer.

The First Substitute flag typically represents a variety of purposes, often relating to the presence or absence of certain officers but not specifically for the Commanding Officer. The Second Substitute and Pennant Nine also have different meanings within naval signaling. Therefore, the importance of the Third Substitute lies in its clear and direct indication of the Commanding Officer's absence, which is crucial for maintaining proper operational protocols and notifying other ships in the vicinity.