INTERNATIONAL ONLY Which signal is sounded ONLY by a vessel in sight of another?

Correct Answer: Option A — One short blast on the whistle

COLREGS Rule 34(a) defines the one short blast signal as a maneuver indicating an alteration of course to starboard. This signal is strictly for vessels in sight of one another. Other signals listed, such as prolonged blasts, are primarily associated with restricted visibility requirements under Rule 35.