BOTH INTERNATIONAL & INLAND You are heading due east (090°) and observe a vessel's red sidelight on your port beam. The vessel may be heading __________.

Correct Answer: Option D — southwest (225°)

Applying relative bearings and COLREGS Rule 21, observing a red sidelight (port) on your port beam while heading due east (090°) places the observed vessel to your south. To present its port sidelight to your east-facing position, that vessel must be heading southwest (225°).