BOTH INTERNATIONAL & INLAND Which whistle signal is required to be sounded by a vessel operating astern propulsion?

Correct Answer: Option C — Three short blasts

Rule 34(a) mandates that when vessels are in sight of one another, a vessel operating astern propulsion shall indicate this maneuver by sounding three short blasts on the whistle. This signal is required to inform other vessels of the change in the vessel's movement and potential change in heading.