Answer
The force on the point charge is exerted at a right angle to the point charge's direction of motion. Therefore, this force does zero work, so the speed of the point charge remains constant.
Work Step by Step
The force on the point charge, which is caused by the magnetic field, is exerted at a right angle to the point charge's direction of motion. Therefore, this force does zero work on the point charge. Since the force does zero work, the kinetic energy of the point charge does not change so the speed of the point charge remains constant.
