Assertion (A): An alpha particle placed in a magnetic field will not experience any force, if it moves in the magnetic field parallel to field lines.
Reason (R): The force is zero if current and field are in the same direction.

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

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that an alpha particle placed in a magnetic field will not experience any force if it moves parallel to the field lines.
- An alpha particle consists of 2 protons and 2 neutrons, giving it a positive charge.
- The force experienced by a charged particle moving in a magnetic field is given by the equation:
\[ F = Q \cdot (v \times B) \]
- Here, \(F\) is the magnetic force, \(Q\) is the charge of the particle, \(v\) is the velocity vector, and \(B\) is the magnetic field vector.
- If the particle moves parallel to the magnetic field lines, the angle (\(\theta\)) between \(v\) and \(B\) is 0 degrees.

2. **Calculating the Force**:
- The formula for the magnetic force can be rewritten as:
\[ F = Q \cdot v \cdot B \cdot \sin(\theta) \]
- Since \(\theta = 0\), we have:
\[ \sin(0) = 0 \]
- Therefore, the force \(F\) becomes:
\[ F = Q \cdot v \cdot B \cdot 0 = 0 \]
- This confirms that the assertion is correct: an alpha particle will not experience any force if it moves parallel to the magnetic field lines.

3. **Understanding the Reason (R)**:
- The reason states that the force is zero if the current and the magnetic field are in the same direction.
- In a current-carrying conductor, the force experienced can also be described by:
\[ F = I \cdot L \times B \]
- Here, \(I\) is the current, \(L\) is the length of the conductor, and \(B\) is the magnetic field.
- If the current direction is the same as the magnetic field direction, the angle between \(L\) and \(B\) is also 0 degrees, leading to:
\[ F = I \cdot L \cdot B \cdot \sin(0) = 0 \]
- This shows that the reason is also correct: the force is zero when the current and the magnetic field are in the same direction.

4. **Conclusion**:
- Both the assertion and the reason are correct.
- Furthermore, the reason provided is a correct explanation of the assertion.

### Final Answer:
- Assertion (A) is correct.
- Reason (R) is correct and provides a valid explanation for Assertion (A).