As the electron is moving perpendicular to the direction of magnetic field, it experiences a maximum force which acts perpendicular to the direction of magnetic field as well as .
This force will provide the required centripetal force for the electron to describe a circular path in the magnetic field of radius r:
Hence we can write that:
Therefore the correct option is 1. v / B
i.e Radius of the circle is proprortional to v / B