As a charged particle moves freely in a circular path in the presence of a constant magnetic field applied perpendicular to the particle's velocity, the particle's kinetic energy:

a. Remains the same
b. Increases
c. Decreases

When a particle moves freely in a circular path resulting from a magnetic field, it is observed that the magnetic force will act in a direction perpendicular to its velocity vector. That is to say that this force will act at 90 degrees of the direction of the velocity of the particle, in this way the displacement of the particle is undergoing. From there it is possible to conclude that the force does not act on the particle and therefore the kinetic energy of this remains constant.

Therefore the correct answser is A.

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c) up is the direction the magnetic force is acting on the charge.