BOTH INTERNATIONAL & INLAND By night, you sight the lights of a vessel engaged in underwater operations. If an obstruction exists on the port side of the vessel, it will be marked by __________.

Correct Answer: Option B — two red lights in a vertical line

COLREGS Rule 27(b) dictates that a vessel engaged in underwater operations marks the side of any obstruction with two red lights in a vertical line, directing vessels to pass clear on the opposite side.