Which portion of the inner ear controls equilibrium?

The vestibular apparatus is the portion of the inner ear responsible for maintaining equilibrium. It consists of semicircular canals that detect rotational movements and otolithic organs (the utricle and saccule) that respond to linear accelerations and the position of the head relative to gravity. These structures work together to provide the brain with information about the body’s orientation and motion, helping to maintain balance and spatial awareness.

In contrast, the cochlea is primarily involved in hearing, converting sound vibrations into neural signals. The basilar membrane is a component within the cochlea where sound wave processing occurs, and it doesn't play a role in balance. The oval window, another structure associated with the cochlea, acts as a membrane that transmits vibrations from the middle ear into the fluid-filled cochlea but has no function in equilibrium. Thus, the vestibular apparatus is specifically designed for balance and spatial orientation, making it the correct answer.