Answer
(d) both (a) and (c) are possible
Work Step by Step
The magnitude of magnetic field on a charged particle with charge q and speed v is
$F=qvBsin\theta$, where $\theta$ is the angle between velocity of the particle and magnetic field B.
In this case, $v\ne0$, $B\ne0$, thus for F=0, we must have $\theta=0^{\circ}$ or $\theta=180^{\circ}$.
Therefore the magnetic field could be in the same or opposite direction .
