Moving electron is just like current flowing in a straight wire and velocity of electron is opposite to the current flow. And according to Biot-savart law:

, where

μ_o is magnetic permeability in vacuum,

q is charge of the particle,

v is constant velocity vector of particle,

and r is the displacement vector for point we want to calculate the magnetic field;

for a charge moving with constant velocity v. So, B will be perpendicular to v and r as B is cross product of v and r.

Hence, our answer is option (a).