Explanation: A continuous watch on VHF Ch-16 is generally required for vessels equipped with a VHF marine radio while underway at sea. This rule is reinforced by several regulations and situations: * **A1 Sea Area:** In a GMDSS A1 sea area, which is within range of shore-based VHF DSC, vessels must maintain a continuous watch on VHF Ch-16 and Ch-70 (for DSC alerting) for distress, urgency, and safety communications. * **Vessel Traffic Service (VTS) System:** Within a VTS system, designated channels for traffic management are typically mandatory, and maintaining a watch on VHF Ch-16 is also often required for general safety and to hear distress calls from other vessels. * **Bridge-to-Bridge Act:** While the Vessel Bridge-to-Bridge Radiotelephone Act specifically mandates a watch on VHF Ch-13 for certain vessels (e.g., commercial vessels 20 meters or more in length), vessels subject to this act are also typically subject to broader GMDSS and FCC regulations requiring a continuous watch on Ch-16. Therefore, option C describes a scenario where a Ch-16 watch is unequivocally compulsory due to the convergence of multiple regulatory requirements and operational conditions. Options A and C are identical in the prompt, implying that the statement represented by C is the correct scenario. Option B is incorrect because a watch on Ch-16 *is* compulsory at most times while at sea for vessels equipped with VHF. Option D is incorrect because a continuous watch on Ch-16 is generally compulsory in all GMDSS sea areas (A1, A2, A3, and A4) unless a vessel is actively using another channel, and then it must return to Ch-16 as soon as practicable.