Frog's eye are protected under water by the presence of

Options

A. Retina
B. Upper eyelid
C. Nictitating membrane
D. Lower eyelid

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks what protects a frog's eyes when it is underwater. Frogs are amphibians, meaning they can live both in water and on land.

2. **Identifying Eye Structures**: Frogs have three types of eyelids:
- Upper eyelid
- Lower eyelid
- Third eyelid (also known as the nictitating membrane)

3. **Analyzing Each Eyelid**:
- **Upper Eyelid**: This eyelid is thick and movable, but it does not help the frog see underwater.
- **Lower Eyelid**: This eyelid is immovable and vestigial, meaning it does not serve a functional purpose in protecting the eyes or aiding vision.
- **Nictitating Membrane**: This is a thin, movable, and transparent eyelid. It is specifically adapted to protect the eyes while allowing the frog to see underwater.

4. **Conclusion**: The nictitating membrane is the structure that protects the frog's eyes when it is submerged in water, allowing for vision while providing protection.

5. **Final Answer**: The correct answer is the **nictitating membrane** (option C).