What characteristic distinguishes AD rivets from other rivets?

AD rivets are characterized by their unique dimple on the head, which is a significant distinguishing feature compared to other types of rivets. This dimple indicates that the rivet is made from aluminum alloy, specifically designated for high strength and corrosion resistance. The unique design allows for better representation of the rivet's intended specifications, particularly in terms of strength and weight.

In the context of other options, while some rivets might have cross marks or plain heads, these features are not exclusive or defining for AD rivets. The size of the rivet, while it might vary, isn't a distinguishing factor either, as rivets can come in various sizes across different types. The dimple thus serves as a clear identifier for AD rivets within the broader categorization of rivets used in aviation and aerospace applications.