Explanation

(a) Drift velocity is defined as the average velocity with which free electrons get defined towards the positive end of the conductor under the influence of an external electric field.
Drift velocity is given by
$v_{d} = \frac{eE\tau}{m}$
$But\ \ \ \ \ E = \frac{V}{l}$
(if $l$ is length of the conductor and V is constant potential difference applied across the ends of the conductor)
$\therefore v_{d} = \frac{eV\tau}{ml} \Longrightarrow v_{d} \propto V$
So, when the potential difference is doubled the drift velocity will be doubled.