Assertion (A): Magnetic field due to current carrying solenoid is independent of its length and cross-sectional area.
Reason (R): Magnetic field is uniform inside the solenoid.

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 also false.

Explanation

To solve the question, we need to analyze both the Assertion (A) and the Reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the magnetic field due to a current-carrying solenoid is independent of its length and cross-sectional area.
- The formula for the magnetic field (B) inside a long solenoid is given by:
\[ B = \mu_0 \cdot n \cdot I \]
where:
- \( \mu_0 \) is the permeability of free space,
- \( n \) is the number of turns per unit length (which is dependent on the total number of turns and the length of the solenoid),
- \( I \) is the current flowing through the solenoid.
- From this formula, we can see that while the magnetic field strength (B) depends on the number of turns per unit length (n) and the current (I), it does not explicitly depend on the total length or the cross-sectional area of the solenoid. Therefore, the assertion is **correct**.

2. **Understanding the Reason (R)**:
- The reason states that the magnetic field is uniform inside the solenoid.
- It is a well-known fact that the magnetic field inside a long solenoid is uniform, meaning that the magnetic field strength is the same at every point inside the solenoid.
- This statement is also **correct**.

3. **Analyzing the Relationship between A and R**:
- While both statements A and R are true, the reason does not provide a correct explanation for the assertion. The assertion is about the independence of the magnetic field from the length and cross-sectional area, while the reason describes the uniformity of the magnetic field inside the solenoid.
- Therefore, we conclude that both A and R are true, but R is not the correct explanation of A.

### Conclusion:
- The correct answer is that both A and R are true, but R is not the correct explanation of A.