(c) If $C$ is capacity of each condenser, then charge on each capacitor$\ = 10\ C$
$(\because V = 10V)$
When connected in series, potential difference between free plates =$\ \frac{total\ charge}{\begin{array}{r}
total\ capacity \\
\
\end{array}}$
$= \frac{10\ C}{C/6} = 60\ V$