c
This can be seen as two capacitors in series combination so
$\frac{1}{C_{e q}}=\frac{1}{C_1}+\frac{1}{C_2}$
$=\frac{1}{\frac{K \in_0 A}{t}}+\frac{1}{\frac{\in_0 A}{d-t}}$
$=\frac{ t }{ K \in_0 A }+\frac{ d - t }{\in_0 A }$
$=\frac{1 \times 10^{-3}}{5 \in_0 \times 40 \times 10^{-4}}+\frac{1 \times 10^{-3}}{\in_0 40 \times 10^{-4}}$
$\frac{1}{ C _{ eq }}=\frac{1}{20 \in_0}+\frac{1}{4 \in_0}$
$C _{ eq }=\frac{20 \times 4 \in_0}{24}=\frac{10 \in_0}{3} F$
