Assertion: Electron moving perpendicular to `vecB` will perform circular motion.
Reason: Force by magnetic field is perpendicular to velocity.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reson 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

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "an electron moving perpendicular to a magnetic field `vecB` will perform circular motion."
- This is based on the principle that a charged particle moving in a magnetic field experiences a magnetic force that acts perpendicular to both the velocity of the particle and the direction of the magnetic field.

2. **Understanding the Reason**:
- The reason states that "the force by the magnetic field is perpendicular to the velocity."
- This is true because the magnetic force on a charged particle is given by the Lorentz force equation:
\[ \vec{F} = q(\vec{v} \times \vec{B}) \]
where `q` is the charge of the particle, `vec{v}` is its velocity, and `vec{B}` is the magnetic field. The cross product indicates that the force is always perpendicular to the velocity.

3. **Analyzing the Motion**:
- When the electron enters the magnetic field perpendicularly, it experiences a magnetic force that continuously changes its direction while maintaining its speed.
- This continuous change in direction, with constant speed, results in circular motion.

4. **Conclusion**:
- Both the assertion and the reason are true. However, the reason does not fully explain how the magnetic field causes the electron to move in a circular path. It only states that the force is perpendicular to the velocity, which is a necessary condition for circular motion but does not explain the role of the magnetic field in creating that force.

5. **Final Answer**:
- Therefore, the correct conclusion is that both the assertion and the reason are true, but the reason is not correctly explaining the assertion.