Explanation:
Explanation:

The semicircular canals are structures in the inner ear that are directly involved in maintaining balance and equilibrium.

How they work:
- There are three semicircular canals arranged at right angles to each other.
- They contain fluid and sensory hair cells.
- When the head moves, the fluid shifts, stimulating the hair cells.
- These signals are sent to the brain, which interprets them to help the body maintain balance and orientation.

---

Let’s examine the other options:

#A. Cochlea
- The cochlea is involved in hearing, not balance. It converts sound vibrations into nerve impulses.

#B. Eustachian tube
- The Eustachian tube helps equalize pressure between the middle ear and the atmosphere. It does not play a direct role in balance.

#C. Ear-drum (Tympanic membrane)
- The ear-drum vibrates in response to sound waves, playing a role in hearing, not balance.

#D. Pinna
- The pinna is the outer ear, responsible for collecting sound waves. It has no role in balance.

Summary:
The semicircular canals are the part of the ear directly responsible for balance, by detecting changes in head movement and position.