What additional signal may be used if the tow is restricted in their ability to deviate from their course?

In circumstances where a tow is restricted in their ability to deviate from their course, a day shape is the appropriate additional signal to indicate this restriction. Specifically, the configuration of a day shape, which consists of a diamond shape for vessels being towed, serves as a visual cue to other mariners that the vessel is constrained in its navigation due to the tow.

This signal is especially important for maintaining safety on the water, as it communicates the conditions of navigation to approaching vessels. Other vessels can then understand that they must take care when passing or maneuvering around the towing vessel, as it may not be able to change its course in a manner typical of an unrestricted vessel.

Sound signals and specific light signals also play crucial roles in maritime communication, but they serve different purposes based on the situations at hand. A distress signal indicates an emergency situation but does not specifically denote a restriction in navigation. Therefore, the use of a day shape is the most appropriate and correct answer in this context.