When entering from seaward, a buoy displaying a single-flashing red light indicates __________.

Correct Answer: Option D — the starboard side of the channel

Under the US IALA Region B system, lateral aids marking the starboard side of the channel when entering from seaward are red, even-numbered, and display red lights. A single-flashing red light thus designates the starboard boundary, aligning with the cardinal rule 'Red, Right, Returning'.