The correct answer is a) cochlea.
The cochlea is a spiral-shaped, fluid-filled structure in the inner ear that plays a crucial role in the auditory system. It contains tiny hair cells, which are the actual receptors for hearing. When sound waves enter the ear, they cause the fluid in the cochlea to move, which in turn stimulates these hair cells. The movement of the hair cells translates sound waves into electrical signals that the brain interprets as sound.
In contrast, the eardrum (b) serves as a barrier between the outer and middle ear and vibrates in response to sound. The semicircular canals (c) and utricle and saccule (d) are part of the vestibular system, which helps with balance and spatial orientation, not hearing.