Explanation:

Given:
- An electron enters a region where the magnetic field (B) and electric field (E) are mutually perpendicular.

Explanation:
When an electron enters a region where both the magnetic field (B) and electric field (E) are mutually perpendicular, the electron experiences the combined effect of both fields. Let's analyze the possible scenarios:

1. The electron moves in the direction of the magnetic field (B):
If the electron initially moves in the direction of the magnetic field (B), it will experience a force given by the Lorentz force equation: F = q(v x B), where q is the charge of the electron, v is its velocity, and x denotes the cross product. The force will cause the electron to move in a circular path around the magnetic field lines.

2. The electron moves in the direction of the electric field (E):
If the electron initially moves in the direction of the electric field (E), it will experience a force given by the equation: F = qE, where E is the electric field strength. In this case, the electron will accelerate in the direction of the electric field.

3. The electron moves perpendicular to both fields:
If the electron initially moves perpendicular to both the magnetic field (B) and electric field (E), it will experience a force due to the combined effect of both fields. The Lorentz force equation can be used to calculate the net force on the electron. Depending on the magnitude and direction of the fields, the electron may be deflected in a curved path or may continue moving undeflected.

Therefore, the correct option is (d) it can go undeflected also. In this case, the electron's initial velocity is perpendicular to both the magnetic and electric fields, resulting in no net force acting on the electron, causing it to move undeflected.

In summary, when an electron enters a region where the magnetic field (B) and electric field (E) are mutually perpendicular, the electron can either move in a circular path, accelerate in the direction of the electric field, or go undeflected depending on its initial velocity and the magnitudes and directions of the fields.