Assertion (A): Excretory unit of kidneys are nephrons.
Reason (R): It has no role in secretion of urine.

Options

A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R is NOT the correct explanation of A.
C. A is true but R is false.
D. A is false and R is true.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "Excretory unit of kidneys are nephrons." This is a true statement, as nephrons are indeed the functional units of the kidneys responsible for filtering blood and forming urine.

2. **Understanding the Reason (R)**: The reason states that "It has no role in secretion of urine." This statement is false. Nephrons play a crucial role in the formation of urine, which includes processes such as filtration, reabsorption, and secretion.

3. **Evaluating the Relationship between A and R**: Since the assertion is true and the reason is false, we can conclude that the reason does not correctly explain the assertion.

4. **Identifying the Correct Option**: Based on the evaluation:
- The first option states both A and R are true, and R explains A (incorrect).
- The second option states both A and R are true, but R does not explain A (incorrect).
- The third option states A is true and R is false (correct).
- The fourth option states A is false and R is true (incorrect).

5. **Conclusion**: The correct answer is that the assertion is true, but the reason is false.

### Final Answer:
The correct option is the third one: Assertion is true but reason is false.