In a cyclotron, a charged particle

Inside the dees in a cyclotron a charged particle changes its direction only and not the speed due to the magnetic field inside it, which applies a force perpendicular to the velocity of the particle. And between the dees due to the very high voltage applied its speed increases due to electric force with no change in direction. Hence, the charged particle undergoes acceleration all the time.

Hence, the correct option is (a).