When a charged particle enters a uniform magnetic field, its kinetic energy: a)increases b)decreases c)becomes zero d)remains constant..give reason.

Kinetic energy remains constant.

Because magnetic force is always perpendicular to the motion of charge particle(ALWAYS)

Therefore work done by magnetic force is  

  F_m.ds = 0    where F_m is magnetic force.

 

Thumb Rule: Work done by magnetic force is always zero,No matter what the system is.

Magnetic force can only change the direction (atmost) of velocity (or motion)  but it can not change magnitude of velocity (never).