d
\(Q = Q_1 + Q_2 = C_1V_1 + C_2V_2 = 100[15 + 1] \times  10^{-6} = 1600\ \mu C\)

\(C_1 = \varepsilon _rC = 15 \times 1\mu F = 15\ \mu F\)

\(C'_1 = 1\ \mu F, C'_2 = 1\ \mu F\)

\(C{'_1} = \frac{{{C_1}}}{{{\varepsilon _r}}} = \frac{{15}}{{15}} = 1\mu F\,\,\,{V_{cm}} = \frac{Q}{{C{'_1} + C{'_2}}} = \frac{{1600 \times {{10}^{ - 6}}}}{{2 \times {{10}^{ - 6}}}} = 800\ V\)