Assertion:At fovea, the visual acuity is the greatest.
Reason:The fovea is a thick area of the retina where both rods and cones are present.

Options

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

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "At fovea, the visual acuity is the greatest." This is a true statement. The fovea is a small pit in the retina that is responsible for sharp central vision. It contains a high density of cone photoreceptors, which are responsible for color vision and visual acuity.

2. **Understanding the Reason**: The reason provided is "The fovea is a thick area of the retina where both rods and cones are present." This statement is incorrect. The fovea is actually a thin area of the retina and is primarily composed of cones, with very few rods present.

3. **Analyzing the Relationship**: Since the assertion is true (the fovea does have the greatest visual acuity), but the reason is false (the fovea is not a thick area and primarily contains cones), we can conclude that the assertion stands independently of the reason.

4. **Final Conclusion**: Therefore, the correct answer is that the assertion is true, but the reason is false.