Which organ in the inner ear is responsible for hearing?

The correct choice is the cochlea, as it plays a crucial role in the process of hearing. The cochlea is a spiral-shaped, fluid-filled structure located in the inner ear. It contains hair cells that act as sensory receptors for sound. When sound waves enter the cochlea through the oval window — a membrane-covered opening separating the middle ear from the inner ear — they create fluid vibrations within the cochlea. These vibrations stimulate the hair cells, converting mechanical energy into electrical signals that are transmitted to the brain via the auditory nerve, ultimately allowing us to perceive sound.

In contrast, the vestibular apparatus is primarily responsible for balance and spatial orientation, not hearing. The oval and round windows are membranes that serve different roles in the transmission of sound and pressure changes within the cochlea, but they do not directly participate in the process of hearing itself. The oval window acts as the entry point for sound waves, while the round window helps to relieve pressure within the cochlea. Thus, understanding the specific functions of these structures clarifies why the cochlea is distinctly associated with hearing.