Where is the sense of balance located in the human body?

The sense of balance, also known as equilibrium, is primarily controlled by the vestibular system.

The vestibular system is located in the inner ear. It consists of the semicircular canals, which detect rotational movements of the head, and the otolith organs (utricle and saccule), which detect linear acceleration and the position of the head relative to gravity. These structures send signals to the brain to help maintain balance and spatial orientation.

• a) Ear: The image shows the ear, including the inner ear structures like the semicircular canals, which are directly responsible for the sense of balance.
• b) Brain: The brain processes the information received from the inner ear and other senses to coordinate balance, but it is not the primary sensory organ for balance itself.
• c) Spine: The spine provides structural support and houses the spinal cord, which transmits signals, but it does not contain the sensory organs for balance.
• d) Feet: The feet contain proprioceptors that contribute to balance by sensing pressure and position, but they are not the primary organ for the sense of balance.

Therefore, the ear, specifically the inner ear, is where the sense of balance is primarily located.

The final answer is $\boxed{\text{a}}$.