The correct option is B Speed
Consider a case when charged particle is entering into magnetic field at right angle. Then the particle performs uniform circular motion. We know in uniform circular motion, only speed is said to be constant; because velocity, momentum, acceleration and force changes their directions continuously.
Similarly no matter at what angle the particle enters the magnetic field. The magnetic force it experience will always be perpendicular to its motion. Hence there will not be any change in the magnitude of its velocity, hence speed remains constant in any case.