What is typically used by ships to produce a loud sound?

The correct choice is a ship's gong, which is a traditional signaling device used on some vessels. While all the options listed produce sound, the ship's gong is often used for communication on board, especially in a maritime context. It provides a loud and resonant noise that can be effective over distances, particularly in situations such as alerting crew members or signaling specific events.

The other options, while also capable of producing sound, serve different primary functions. A whistle is used for signaling at close range and may not have the same loudness and reach as a gong. A fog horn, while designed to emit loud sounds for navigation safety in foggy conditions, is typically stationary and not used for signaling between ships or onboard communication in the same way as a gong. A signal cannon, although loud, is used mainly for ceremonial purposes or to mark specific times or events rather than for routine communication among crew members. The ship's gong stands out for its utility in producing a clear and loud signal during various maritime situations.