What is resonance condition in cyclotron? How is it used to accelerate the charged particles ?

Cyclotron frequency should be equal to the applied voltage is the resonance condition!
n = qB/2πm = applied voltage.
​And also do remember that ​cyclotron Frequency is independent from velocity as well as radius of the path followed by the particle.
 

A cyclotron is a particle accelerator in which the charged particles accelerate on a spiraled path which is directed outwards from the center. The particles are held in their path by a static magnetic field and an accelerating electric field.

A synchrotron is also a particle accelerator, very much similar to a cyclotron, in which the magnetic field is time-dependent and is synchronized to the beam of accelerating particles. There is no electric field used in a synchrotron.

Cyclotron and synchrotron are two different devices. However, a cyclotron with no electric field and changing magnetic field will behave like a synchrotron.