What fog signal equipment is required on a vessel that is 20 meters in length?

For a vessel that is 20 meters in length, the requirement under the International Regulations for Preventing Collisions at Sea (COLREGs) stipulates that it must have both a whistle and a bell. This is essential for ensuring that the vessel can effectively signal its presence and location in foggy conditions where visibility is significantly reduced.

The whistle serves as a primary means of making a sound signal in conditions of reduced visibility, while the bell is used to indicate the vessel's presence, particularly when the vessel is at anchor. The combined use of these sound signaling devices is crucial for safety, as it helps to prevent collisions by alerting other vessels in the vicinity of the vessel’s location.

Other options that suggest having only one type of sound signaling device, such as just a whistle or just a bell, do not meet the regulatory requirements for vessels of this size. Additionally, while "sound signal device only" suggests a general category, it does not specify the necessary types of signals required for a vessel of this length. Hence, the requirement for both a whistle and a bell is clear in the regulations for vessels measuring 20 meters.