If cathode rays enter a magnetic field making an oblique angle with lines of induction, then their path in the magnetic field is
A. A straight line
B. Helix
C. A circle
D. Parabola

Hint: In order to solve this question we should know the concept that when the cathode ray which enters the magnetic field.
The magnetic force is perpendicular to the direction of motion, then a charged particle follows a circular path in a magnetic field.

Complete answer:
The Magnetic force is responsible for a charged particle to move in a circular motion. The charge particles are forced to move spiral motion by Earth’s magnetic field. The Protons in acceleration are kept in the circular motion by force due to the magnet. The charged particles move in circular motion in the magnetic fields depending upon the number of phenomena and they are used analytically such as in a spectrometer.
In a simple way to explain, when a charged particle travels perpendicular in a uniform magnetic field. If the magnetic field is in the state of vacuum, then the magnetic field is the factor determining the motion. Since the magnetic force is perpendicular to the direction of motion, then a charged particle follows a circular path in a magnetic field. The particle motion which continues in the same path until complete full cycle.
The Magnetic force which is always perpendicular to its velocity, therefore there is no work done on the charged particle which is in motion. The kinetic energy and the speed of the charge particle remain constant. But there is no effect on speed, only its effect on the direction of motion. This is typical of uniform circular motion.
Therefore the Charge particles move in a circular motion which is perpendicular to the magnetic field.

So, the correct answer is “Option C”.

Note:
If the velocity is not perpendicular to the magnetic field then we can compare the both components of velocity with respect to the direction of the magnetic field. If both magnetic field and velocity are perpendicular to each other, then the force experienced by the charge is also perpendicular to both.