Final answer:
The answer to which option is not a type of membrane is fibrous. Synovial, serous, and mucus are all types of membranes, while fibrous refers to a type of joint.
Step-by-step explanation:
The student has asked which of the following is not a type of membrane: fibrous, serous, synovial, or mucus. In biology, there are two broad categories of tissue membranes in the body, which are connective tissue membranes and epithelial membranes. The connective tissue membranes include synovial membranes, which line the cavities of freely movable joints.
Epithelial membranes include mucous membranes, containing glands; serous membranes, which secrete fluid; and cutaneous membranes, which make up the skin.
From the options provided, the answer would be fibrous as it is not listed as a type of tissue membrane. Fibrous is actually a type of joint (fibrous joint), not a membrane. Therefore, fibrous is the correct choice for the type of structure that is not a membrane.