d
(d)
Potential due to an elemental charge $d Q$ at centre of sphere is
$d V=\frac{k d Q}{R}$
Potential due to complete charge distribution at centre is
$V=\oint d V=\oint \frac{k d Q}{R}$
$=\frac{k}{R} \oint d Q=\frac{kQ}{R}$
or
$V=\frac{k Q}{R}=\frac{Q}{4 \pi \varepsilon_0 R}$