Assertion : When a convex lens `(mu_(g) = 3//2)` of focal length f is dipped in water, its focal length become `(4)/(3)f`
Reason : The focal length of convex lens in water becomes 4f.

Options

A. Assertion is correct, reason is correct, reason is a correct explanation for assertion.
B. Assertion is correct, reason is correct, reason is not a correct explanation for assertion
C. Assertion is correct, reason is incorrect
D. Assertion is incorrect, reason is correct

Explanation

To solve the given question, we need to analyze the assertion and the reason provided regarding the focal length of a convex lens when it is dipped in water.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- The assertion states that when a convex lens with a refractive index \( \mu_g = \frac{3}{2} \) and focal length \( f \) is dipped in water, its focal length becomes \( \frac{4}{3}f \).

2. **Understanding the Reason:**
- The reason states that the focal length of the convex lens in water becomes \( 4f \).

3. **Using the Lens Maker's Formula:**
- The lens maker's formula for a lens in a medium is given by:
\[ \frac{1}{f} = \left( \frac{\mu_2}{\mu_1} - 1 \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
- Here, \( \mu_2 \) is the refractive index of the lens material, \( \mu_1 \) is the refractive index of the surrounding medium, and \( R_1 \) and \( R_2 \) are the radii of curvature of the lens surfaces.

4. **Calculating Focal Length in Air:**
- In air, the refractive index \( \mu_1 = 1 \) and \( \mu_2 = \frac{3}{2} \):
\[ \frac{1}{f} = \left( \frac{3/2}{1} - 1 \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
\[ \frac{1}{f} = \left( \frac{1}{2} \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]

5. **Calculating Focal Length in Water:**
- When the lens is dipped in water, the refractive index of water \( \mu_1 = \frac{4}{3} \):
\[ \frac{1}{f'} = \left( \frac{3/2}{4/3} - 1 \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
\[ \frac{1}{f'} = \left( \frac{9}{8} - 1 \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]
\[ \frac{1}{f'} = \left( \frac{1}{8} \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \]

6. **Relating Focal Lengths:**
- Now, we can relate the two equations:
\[ \frac{1}{f'} = \frac{1}{f} \cdot \frac{8}{2} = \frac{4}{f} \]
- Therefore, \( f' = 4f \).

7. **Conclusion:**
- The assertion is incorrect because the focal length does not become \( \frac{4}{3}f \); it becomes \( 4f \) as stated in the reason. Thus, the reason is correct.

### Final Answer:
- The assertion is false, and the reason is true. Therefore, the correct option is that the assertion is incorrect while the reason is correct.