What feature distinguishes a wet suit?

The distinguishing feature of a wet suit is that it allows a thin layer of water to enter, which is then warmed by the diver's body heat. This concept is fundamental to how wet suits function. The neoprene material used in the suit provides insulation against the cold water while the layer of water trapped between the diver's body and the suit is warmed to a comfortable temperature. This mechanism keeps the diver warm and allows for flexibility and ease of movement in the water while still providing thermal protection.

Other options do not accurately reflect the characteristics of a wet suit. For example, a wet suit does not keep the diver completely dry; rather, it is designed to get wet. Additionally, while it may be lighter than a dry suit, the weight and thickness compared to materials do not solely identify it as a wet suit. Thus, option C correctly captures the essential feature that makes a wet suit distinct from other types of thermal protection gear used in diving.