Which flag is flown inboard at the port yardarm in the absence of the chief of staff?

The correct answer is the second substitute flag. This flag is used to indicate that the chief of staff is absent from a ship or unit, thereby following the appropriate protocol in naval customs. When the chief of staff is not present, the second substitute is flown at the port yardarm in order to inform those observing, both on board and from the outside, about the temporary change in command or duty status.

This practice is rooted in the traditions of naval signaling which serve to communicate important operational statuses. The flying of the second substitute flag effectively conveys vital information regarding personnel availability and operational readiness, ensuring that all involved are informed of the situation.

In this context, other flags would not be suitable as they serve different purposes. For example, the first substitute flag indicates the absence of a commanding officer, the help flag is used to signal for assistance, and the emergency flag is raised in situations requiring urgent attention or signaling a distress situation. Therefore, the second substitute is the appropriate choice when denoting the absence of the chief of staff specifically.