Which of the following signifies that a vessel is operating astern propulsion?

Answer: C — One long blast

A vessel operating astern propulsion is indicated by one long blast. This signaling is critical for communication between vessels navigating in close proximity to each other, particularly in congested or narrow waterways. The sound of one long blast serves to inform other vessels of the maneuver the vessel is about to undertake, which is essential for safety and to prevent collisions. The use of sound signals is part of the COLREGs (International Regulations for Preventing Collisions at Sea) governing the conduct of vessels at sea. Understanding these signals helps mariners communicate their intentions effectively, particularly in situations where visibility may be reduced. When a vessel is operating astern propulsion, the sound signal of one long blast helps other operators recognize that it is moving backward, allowing them to take necessary precautions. Other options like two short blasts, two long blasts, and one short blast correspond to different maneuvers or intentions. For instance, two short blasts indicate that a vessel is altering course to starboard, while two long blasts signify a maneuver in a restricted visibility situation. Similarly, a single short blast typically indicates a course alteration to port. Therefore, understanding these distinctions is important for safe navigation and compliance with maritime regulations.