What may be used to indicate the presence of a partly submerged object being towed?

The option indicating the beam of a searchlight shown in the direction of the tow is correct as it serves a specific purpose in navigation safety. When a vessel is towing an object that may not be easily seen, such as a partly submerged object, using a powerful beam of light helps to illuminate the area for other vessels, thereby increasing visibility. This is especially important during nighttime or in low visibility conditions, ensuring that the towed object is marked to prevent collisions.

Utilizing a searchlight effectively alerts nearby vessels to the potential danger of the tow, allowing them to navigate safely around it. This action aligns with the conventions put forth in navigation rules to prevent accidents and enhance safety on the water.