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

The correct understanding revolves around the specific sound signals that are permitted under the Rules of the Road, which govern navigation on the waters. These rules stipulate distinct sound signals that vessels must use to communicate their intentions, particularly under conditions of restricted visibility or to avoid collisions.

The sounds associated with letter C, namely D, E, and S, refer to signals designated for specific navigational situations, including maneuvers such as sounding a signal when operating in a narrow channel or when overtaking another vessel. These signals align directly with the requirements established in the Rules of the Road, which are crucial for maintaining safety and proper communication on the water.

The other options encompass combinations that include sounds not exclusively governed by the Rules of the Road or that might relate to operational signals outside the prescribed navigational guidelines. Thus, D, E, and S distinctly pertain to the regulations set out for safe navigation, confirming their sole compliance with the established rules.