c
$(c)$ From $C=\frac{q}{V}$
We have, $V=\frac{q}{C}=\frac{q d}{\varepsilon_{0} A}$ or $V \propto d$
Hence, graph is a straight line.
When both plates are touched $(d=0)$,
system still have some capacity $(C \neq 0)$.
So, from $C=\frac{q}{V}$ we can see that at very small seperations $V \neq \infty$ or graph does not have any values for very small $d$.
$\therefore V$ versus $d$ graph is as shown below :
