Focal length of the eye lens can be adjusted by action of :

Options

A. ciliary muscles
B. choroid
C. optical nerves
D. retina

Answer

**Step-by-Step Solution:**

1. **Understanding the Eye's Structure**: The human eye contains a lens that focuses light onto the retina. The lens is flexible and can change its shape to adjust the focal length.

2. **Role of the Ciliary Muscles**: The ciliary muscles are attached to the eye lens. These muscles play a crucial role in adjusting the shape of the lens.

3. **Mechanism of Focal Length Adjustment**:
- When the ciliary muscles contract, they pull on the lens, making it thicker. A thicker lens has a shorter focal length, allowing the eye to focus on nearby objects.
- When the ciliary muscles relax, the lens becomes thinner, which increases the focal length, enabling the eye to focus on distant objects.

4. **Conclusion**: The focal length of the eye lens is adjusted by the action of the ciliary muscles. Therefore, the correct answer to the question is "ciliary muscles".

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