Amembrane that lines a cavity with an opening to the external environment is called a membrane. a. synovial b. cutaneous c. mucus d. serous

C. mucu

Explanation:

Mucous membranes are responsible for the internal lining of the moist body cavities that maintain contact with the external environment such as the mouth, intestine, bladder, respiratory tract and urogenital tract.  

Because they are subject to microbial invasions, the mucous membranes of these regions have accumulations of lymphocytes in the form of lymph nodes associated with diffuse lymphatic tissue. In some regions, such as the ileum, lymphatic tissue is organized into well-structured organs called tonsils and Meyer's plaques.