c
(c)
Work done in the process $=$ Potential energy of the system
Also from shell theorem, charge $Q$ on shell behaves as a point charge at centre. So, magnitude of work done is
$\left|U_{\text {system }}\right| =\left|U_{12}+U_{23}+U_{31}\right|$
$=\mid \frac{k Q q}{a / 2}+\frac{k q(-q)}{a}+\frac{k Q(-q)}{a / 2}$
$=\left|\frac{k}{a}\left(2 Q q-q^2-2 Q q\right)\right|$
$=\left|\frac{-q^2}{4 \pi \varepsilon_0 a}\right|=\frac{q^2}{4 \pi \varepsilon_0 a}$
