When a force of constant magnitude acts perpendicular to the motion of a particle then - 

A) Velocity is constant                      B) Acceleration is constant 

C) KE is constant                              D) None of the above

A). velocity is constant...:))

When force is acting perpendicular to a particle's motion,

F is perpendicular to displacement (s).

Work = F.s (dot product of force and displacement)

= Force x displacement x cos90^o

but cos 90^o = 0

so, work done is 0.

but work is the change in kinetic energy

so change in KE =0

so, C) KE is a constant