(c) Potential at a point in a field is defined as the amount of work done in bringing a unit positive test charge, from infinity to that point along any arbitrary path, $i.e.,$
$V = \frac{W}{q_{0}}$
$\therefore\ \ \ V = \phi = \frac{W}{\mathcal{Q}}\ \ \ \ \ \ \ \ \ \ \ \ \ (\because X \ll \infty)$