According to question, a charge Q is moving d distance in magnetic field B. As we know that force acting on a point charge by magnetic field is always perpendicular to the velocity of particle v. ∴F=Q(v×B).....(i) The above, statement means that F is perpendicular to v. ∵ Work done, W=F⋅ds =Q(v×B)⋅dl [using Eq. (i)] =Q(v×B)dlcosθ =Q(v×B)dlcos90∘[∵θ=90∘F⟂v or dl] =0 ∴ The work done by the magnetic field will be zero.