The force on the electron is given by the Lorentz equation
The proton's velocity after being accelerated through a potential difference in the direction is
We are told that the electric field is in the direction and that the magnetic field is in the direction. Therefore,
Because the proton's trajectory is unaffected, we set in equation (1).
So, initially . But when the potential difference is increased by a factor of , is increased by a factor of two (see equation (2)). Therefore, , so the particle is deflected in the direction. Therefore, answer (B) is correct.