BOTH INTERNATIONAL & INLAND What does a light signal of three flashes mean?

Correct Answer: Option C — "I am operating astern propulsion"

Rule 34(a) specifies that when vessels are in sight of one another, a vessel operating astern propulsion shall indicate this by sounding three short blasts on the whistle. This may be supplemented by an appropriate light signal of three flashes, corresponding to the whistle signal, to ensure clear communication.