Question

what do saliva, tears, and nasal mucus have in common?
all can be classified as biological barriers.
all contain langerhans cells.
all can destroy bacterial cell walls.
all are part of the complement system.

Explanation:

Saliva, tears, and nasal mucus all contain lysozyme. Lysozyme is an enzyme that can destroy bacterial cell walls by hydrolyzing the peptidoglycan layer in the cell wall. Biological barriers refer to physical structures like skin. Langerhans cells are in the skin. The complement system is a part of the immune system involving proteins in the blood, not directly related to these secretions in the way described in the other options.

Answer:

All can destroy bacterial cell walls.