Assertion : An electron is not deflected on passing through certain region of space. This observation confirms that there is no magnetic field in that region.
Reason : The deflection of electron depends on angle between velocity of electron and direction of magnetic field.

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If assertion is false but reason is true

Explanation

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

**Assertion:** An electron is not deflected on passing through a certain region of space. This observation confirms that there is no magnetic field in that region.

**Reason:** The deflection of the electron depends on the angle between the velocity of the electron and the direction of the magnetic field.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- The assertion states that an electron is not deflected in a certain region of space. This implies that there are no forces acting on the electron that would cause it to change its direction.
- According to the principles of electromagnetism, an electron will not be deflected if there is no magnetic field present or if the magnetic field is oriented in such a way that it does not exert a force on the electron.

2. **Analyzing the Reason:**
- The reason provided states that the deflection of an electron depends on the angle between its velocity and the direction of the magnetic field.
- The force acting on a charged particle moving in a magnetic field is given by the Lorentz force equation: \( F = q(\mathbf{v} \times \mathbf{B}) \), where \( q \) is the charge of the particle, \( \mathbf{v} \) is its velocity, and \( \mathbf{B} \) is the magnetic field.
- The cross product \( \mathbf{v} \times \mathbf{B} \) indicates that the force (and thus the deflection) is dependent on the sine of the angle between the velocity vector and the magnetic field vector. If the angle is 0° (parallel) or 180° (anti-parallel), the sine is 0, resulting in no force and hence no deflection.

3. **Conclusion:**
- The assertion is true because if the electron is not deflected, it suggests that there is no magnetic field present.
- The reason is also true because the deflection indeed depends on the angle between the velocity of the electron and the magnetic field.
- Since both the assertion and reason are true, the assertion supports the reason.

### Final Answer:
Both the assertion and reason are true, and the assertion correctly explains the reason.