Assertion- Cyclotron doesn’t accelerate electrons
Reason- Mass of the electrons is very small.
A. Both Assertion and Reason are correct and the Reason is the correct explanation for assertion.
B. Both Assertion and Reason are correct but the Reason is not the correct explanation for Assertion.
C. Assertion is correct but the Reason is incorrect.
D. Both Assertion and the Reason are incorrect.

Hint- As per Einstein's special theory of relativity, the mass of the charged particle should increase as the speed of the particle is increasing. Thus, the frequency of the given cyclotron will always decrease as the mass is increasing. Applying this theory, we get-

Complete step-by-step answer:
A cyclotron is a kind of particle accelerator invented by Ernest O. Lawrence at the University of California, Berkeley, in 1929–1930, and patented in 1932. A cyclotron accelerates charged particles along a spiral path outward from the center. The particles are guided by a static magnetic field to a spiral trajectory, and accelerated by a rapidly changing electric field (radio frequency). For this invention Lawrence received the 1939 Nobel Prize in Physics.
Cyclotrons were the most efficient particle accelerator equipment until the 1950s, when the synchrotron replaced them, and are now used in science and nuclear medicine to generate particle beams.
This will toss the particle out of resonance with the oscillating field which means that when the particle reaches the hole between the dees, the extremity of the dees won't change. Thus, the particle won't be accelerated further.
So, Cyclotron isn't reasonable for accelerating electrons. Because of the small mass, the speed of electrons increments quickly. Similarly, because of fast relativistic variation in their mass, the electrons escape step with the oscillating electric field.
Hence, option A is the correct option.

Note: Cyclotron can be characterized as a kind of particle accelerator where charged particles quicken outwards from the inside along a spiral path. These particles are held to a spiral trajectory by a static magnetic field and quickened by a quickly shifting electric field.