A proton, a deuteron and an $\alpha$ particle accelerated through the same potential difference enter a region of uniform magnetic field, moving at right angles to B. What is the ratio of their K.E.?

K.E. gained by charged particle of charge q when accelerated under a potential difference V will be ${\mathrm{E}}_{\mathrm{k}}=\mathrm{qV}$.

For a given V, $E\propto q$.
For proton, deuteron and $\alpha$ -particle, the ratio of charges is 1: 1: 2