A particle of charge q and mass m starts moving from the origin under the action of an electric field  E→=E0i^ and magnetic field B→=B0i^ with a velocity v→=v0j^. The speed of the particle will become 2v0  after a time.

1. Magnetic field can’t change speed of charged particle, it will only change the direction of  velocity v0j^  such that it always lie in y-z  plane.2. Since the electric field is in  x-direction, hence it would not affect the velocity in the y-z plane.      After time  t, the electric field will inject some velocityvx in the x- direction such that the magnitude of velocity changes to  2v0.  ∴v02+vx2=2v0⇒vx2=4v02−v02=3v02⇒vx=3v03. Electric force  F=qE04. Acceleration  =Fm5.  The time taken by x-component velocity to become from zero to 3v0 due to uniform electric field  =vxfinal−vxinitialacceleration=3v0−0a      here  a=E0qmt=3v0E0qm=3v0mE0qTherefore, the correct answer is D