Question

the membranous labyrinth contains fluid. cerebrospinal fluid plasma perilymph mucus endolymph

Explanation:

The membranous labyrinth is part of the inner ear. It is filled with endolymph. Cerebrospinal fluid is in the central nervous system, plasma is the liquid part of blood, perilymph surrounds the membranous labyrinth (not inside it), and mucus is a secretion in various body tracts (like respiratory).

Answer:

endolymph