The structure that helps a person to maintain his balance is

Options

A. middle car
B. eustachain tube
C. cochlea
D. semi-circular canals

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the structure that helps a person maintain balance, which is related to the concept of equilibrium in the body.

2. **Identifying Relevant Body Parts**: The structure responsible for balance is located in the ear, specifically in the inner ear.

3. **Differentiating Between Structures**: The inner ear contains two important systems: the cochlea and the vestibular apparatus. The cochlea is primarily involved in hearing, while the vestibular apparatus is responsible for balance.

4. **Components of the Vestibular Apparatus**: The vestibular apparatus consists of three semicircular canals and two sacs known as the saccule and utricle. These structures work together to help maintain balance.

5. **Function of the Semicircular Canals**: The three semicircular canals are oriented at right angles to each other and are filled with a fluid called endolymph. They detect changes in movement (acceleration and deceleration) and position.

6. **Mechanism of Balance**: When the head moves, the endolymph fluid in the canals shifts, causing a structure called the cupula to move. This movement bends hair cells (cilia) within the canals, generating nerve impulses that inform the brain about the position and motion of the head.

7. **Conclusion**: Based on the functions of these structures, the semicircular canals are the primary components that help maintain balance in a person.

8. **Final Answer**: Therefore, the structure that helps a person maintain balance is the **semicircular canals**.