Which single-letter sound signal may be made only in compliance with the International Rules of the Road?

The single-letter sound signal "D" signifies that a vessel is "making way through the water." This signal is specifically utilized in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs). The use of sound signals is critical for communication between vessels, especially in conditions of reduced visibility, where visual signals may not be adequate.

When a vessel makes this sound signal, it indicates to other nearby vessels that it is maneuvering and navigating in the water. This is vital for ensuring safety and preventing collisions. It is important for vessels to understand and correctly interpret these sound signals, as they help clarify a vessel's status and intentions.

The other sound signals pertaining to the letters A, B, and C do not denote the act of making way through the water and are associated with different meanings within the same regulatory framework. As a result, the distinct nature of the "D" signal makes it the correct choice in compliance with the International Rules of the Road.