Final Answer:
The portion of the ear essential for hearing and equilibrium is the Inner ear. Option D is the answer.
Step-by-step explanation:
The inner ear (option d) is a critical component responsible for both hearing and equilibrium. It consists of the cochlea, which is vital for hearing, and the vestibular system, which contributes to balance and spatial orientation. The cochlea contains sensory cells that transform sound vibrations into electrical signals sent to the brain, facilitating the sense of hearing. Simultaneously, the vestibular system, located in the inner ear, helps maintain balance and spatial orientation by detecting changes in head movement. The middle ear (option a) and eardrum (option b) are involved in the hearing process, while the external ear (option c) collects sound waves, directing them toward the eardrum.
Option D is the answer.