a
(a) As the given combination is in series
$\frac{1}{C_{e q}}=\frac{1}{C_{1}}+\frac{1}{C_{2}}+\frac{1}{C_{3}}$
$=\frac{d_{1}}{K_{1} \in_{o} A}+\frac{d_{2}}{K_{2} \in_{o} A}+\frac{d_{3}}{K_{3} \in_{o} A}$
$\frac{1}{C_{e q}}=\frac{1}{\in_{o} A}\left[\frac{d_{1}}{K_{1}}+\frac{d_{2}}{K_{2}}+\frac{d_{3}}{K_{3}}\right]$
$C_{e q}=\frac{\in_{o} A}{\frac{d_{1}}{K_{1}}+\frac{d_{2}}{K_{2}}+\frac{d_{3}}{K_{3}}}$