(d) $C = \frac{A\varepsilon_{0}}{d} = 10\mu F$
$C_{1} = \frac{A\varepsilon_{0}}{d - t + \frac{t}{k}} = \frac{A\varepsilon_{0}}{d - \frac{d}{2} + \frac{d}{2k}} = \frac{A\varepsilon_{0}}{\frac{d}{2}\left( 1 + \frac{1}{2} \right)} = \frac{4}{3}.\frac{A\varepsilon_{0}}{d}$
$\therefore C_{1} = \frac{4}{3} \times 10 = 13.33\mu F$