A proton is moving past you horizontally from left to right. Without knowing anything about the direction of the magnetic field, which of the following forces could act on the proton as a result?
A. a force to the left
B. a force upward
C. a force up and to the left
D. Either B or C, but not A
E. Either A, B, or C
Well, A is out, as magnetic field will be zero if in the same direction as velocity.
B is possible, which could counteract a force of gravity on the particle.
C. is possible
so, D is the best choice.