The correct answer is C) Seeing both sidelights of a vessel directly ahead. This is the correct answer because in a head-on situation, where two vessels are approaching each other directly, the observer will see the sidelights (red and green) of the other vessel. This is in accordance with the International Regulations for Preventing Collisions at Sea (COLREGS), which govern navigation rules both internationally and inland. The other options are incorrect because: A) seeing both sidelights off the starboard beam indicates the vessel is crossing your path, not approaching head-on; B) seeing two white towing lights indicates the vessel is towing, not in a head-on situation; and D) seeing only one red light indicates the vessel is to your port side, not directly ahead.