Which statement accurately describes the requirement for an active sonar system to detect submarines?

An active sonar system works by emitting sound waves and then listening for their echoes to detect objects underwater, such as submarines. For a submarine to be detected using active sonar, it must produce a noise that can be picked up by the sonar system. This could be any noise made by the submarine, whether from its engines, movement through the water, or any other operational sound. The active sonar will reflect sound waves off the submarine, allowing it to be located.

In contrast, the other statements do not accurately capture the nature of active sonar detection. For example, requiring a submarine to maintain a continuous surface presence or be within visual range does not align with the operational capacity of sonar, which is designed to function underwater without needing to see the target. Likewise, while submarines do have specific frequencies they might operate on, active sonar does not necessitate that the submarine use a specific frequency for it to be detected; rather, it relies on detecting any noise made by the submarine.