The head of a tapeworm is known as the "scolex". The scolex is the attachment organ of the tapeworm that enables it to attach to the host's intestinal wall. It is a small, flattened structure that contains suckers and hooks, which the tapeworm uses to hold onto the intestine. The rostellum is a structure found on some tapeworms that helps them attach to the host's intestinal wall, but it is not the same as the scolex. Therefore, the correct answer is "scolex" when referring to the head of a tapeworm.