In a region of constant potential:
Constant
potential $$\Rightarrow \mathrm{dV}=0$$ so by relation $$E=\dfrac{-d V}{d r},
\mathrm{E}=0$$
i.e., the
E.F. is not uniform discards answer (a) and agree which answer (b).
As potential
may be outside the charge also so there can be no charge inside the region of
constant potential. It verifies answer (c).
If a charge
is placed in outside region, potential difference in region will not be changed
or electric field will not be changed. It makes answer (d) false.