The correct answer is D) a diamond shape. According to the Navigation Rules, International and Inland, Rule 24(g), a partly submerged vessel or object being towed that is not readily visible must display "a diamond shape at or near the aftermost part of the vessel or object being towed." This diamond shape signals to other vessels in the area that there is an obstruction or hazard that needs to be avoided. The other answer choices are incorrect because they do not align with the specific requirements for marking a partly submerged towed object. Red lights, a black ball, and yellow lights would not provide the necessary visual cue for a partly submerged and not readily noticeable object.