Explanation: Vessels equipped with VHF-FM radios are required by international regulations to monitor Channel 16 at all times when at sea. Channel 16 (156.8 MHz) is designated as the international distress, safety, and calling frequency for maritime VHF radio. This rule ensures that all vessels can hear distress calls from other ships or shore stations and can be reached for urgent communications. While other channels like Channel 8 (intership safety) or specific working channels (e.g., 5A, 1A for port operations or commercial traffic) have their uses, Channel 16 is the primary frequency for initiating contact and broadcasting alerts, making continuous monitoring essential for maritime safety.