When the force acting upon the body is always perpendicular to the velocity, it means there is no displacement in the direction of force, it means there is no work done on the particle. Hence the kinetic energy of the particle remains unchanged. Hence option C is correct. Since the particle is acted upon by a force, its velocity must change according to Newtons Second Law of motion and this change must be in the direction of the force. Since the force is always perpendicular to the velocity it only changes its direction but not the magnitude. Thus, it results in the movement of the particle in a circular path in which the force is always along the radius. Hence option d is also correct.