Answer:
The kinetic energy will remain constant
Explanation:
Charged particles in a magnetic field feel a force perpendicular to their velocity, since the force is F = qvB, and a charged particle feels a force of constant magnitude always directed perpendicular to its motion which causes circular motion in the magnetic field but the speed and kinetic energy of the particle will remain constant.
Therefore, if only the magnetic field acts on the particle, its kinetic energy will remain constant.