Assertion : If a convex lens of glass is immersed in water its power decreases.
Reason : In water it behaves as a convex lens.

Options

A. If both the assertion and reason are true statement and reason is correct explanation of the assertion .
B. If both the assertion and reason are true statement but reason is not a correct explanation of the assertion .
C. If the assertion is true but the reason is a false statement.
D. If both assertion and reason are false statements.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "If a convex lens of glass is immersed in water, its power decreases."
- The power (P) of a lens is given by the formula:
\[ P = \frac{1}{f} \]
where \( f \) is the focal length of the lens. The focal length of a lens depends on the refractive index of the lens material and the medium in which it is placed.

2. **Refractive Index Consideration**:
- The refractive index of glass (n_glass) is typically greater than that of water (n_water).
- When the lens is in air, the refractive index of the lens is compared to that of air (n_air). However, when the lens is immersed in water, the effective refractive index changes.

3. **Calculating Effective Focal Length**:
- The formula for the focal length of a lens in a medium is given by:
\[ \frac{1}{f} = (n_{lens} - n_{medium}) \cdot \frac{1}{R} \]
where \( R \) is the radius of curvature of the lens.
- When the lens is in water, the effective refractive index becomes \( n_{glass} - n_{water} \), which is less than \( n_{glass} - n_{air} \).
- As a result, the focal length \( f \) increases, leading to a decrease in power \( P \).

4. **Understanding the Reason**:
- The reason states that "In water it behaves as a convex lens."
- This is true; a convex lens remains a convex lens regardless of the medium it is in. However, the power of the lens is affected by the refractive indices of the lens and the surrounding medium.

5. **Conclusion**:
- The assertion is correct because the power of the lens decreases when immersed in water.
- The reason is also correct, but it does not explain why the power decreases. It simply states that the lens behaves as a convex lens, which is true but does not address the change in power.

### Final Answer:
- Both the assertion and reason are correct, but the reason is not the correct explanation for the assertion.