Assertion : Magnetic field is useful in producing parallel beam of charged particle.
Reason : Magnetic field inhibits the motion of charged particle moving across it.

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.

Answer

To solve the question regarding the assertion and reason about the magnetic field and charged particles, we can break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Magnetic field is useful in producing parallel beam of charged particles."
- This is true because a magnetic field can influence the motion of charged particles, causing them to move in a controlled manner. When charged particles move through a magnetic field, they experience a force that can be used to align them into a parallel beam.

**Hint**: Think about how charged particles behave in a magnetic field and how their paths can be manipulated.

2. **Understanding the Reason**:
- The reason states that "Magnetic field inhibits the motion of charged particles moving across it."
- This is also true. When a charged particle moves through a magnetic field, it experiences a magnetic force (given by the Lorentz force law: F = q(v × B)), which acts perpendicular to its velocity. This force does not do work on the particle but changes its direction, effectively "inhibiting" its straight-line motion.

**Hint**: Recall the right-hand rule and how the magnetic force acts on a moving charge.

3. **Analyzing the Relationship**:
- The assertion and reason are both true statements. However, we need to determine if the reason correctly explains the assertion.
- The assertion is about the utility of the magnetic field in producing a parallel beam, while the reason describes the behavior of charged particles in a magnetic field. The reason does not directly explain how the magnetic field produces a parallel beam but rather describes the effect of the magnetic field on the motion of charged particles.

**Hint**: Consider whether the reason provides a direct explanation for the assertion or if they are related but separate concepts.

4. **Conclusion**:
- Both the assertion and the reason are true, but the reason is not a correct explanation for the assertion. Therefore, the correct answer would be that the assertion is true, the reason is true, but the reason does not correctly explain the assertion.

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