When the magnetic force acts on a charged particle, change occurs in:

Explanation

The magnetic force acts on a charged particle moving in a magnetic field, which is given by the equation F = q(v × B), where F is the magnetic force, q is the charge, v is the velocity, and B is the magnetic field. The force is always perpendicular to both the velocity of the particle and the magnetic field, which results in a change in the particle's direction of motion but not its speed. Thus, the correct answer is that only the direction of velocity changes. Option A is incorrect because it suggests that only the magnitude changes, which is not the case. Option B incorrectly states that both magnitude and direction change, which also does not happen. Option D is incorrect because it ignores the directional change caused by the magnetic force.