The correct option is D Vestibule
The membranous labyrinth is filled with fluid called endolymph.
The component parts of the membranous labyrinth are the three semicircular ducts, each with a sense organ for rotational movement; the cochlear duct, with the sense organ for sound vibrations, and in the vestibule, the utricle and saccule, with their two sense organs for linear movement.
Within the bony labyrinth is a membranous labyrinth, which is also divided into three parts: the semicircular ducts; two saclike structures, the saccule and utricle, located in the vestibule; and the cochlear duct, which is the only part of the inner ear involved in hearing.
Vestibule is a part of bony labyrinth.