Explanation

(d) In series arrangement potential difference is the sum of the individual potential difference of each capacitor.
$ie,\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ V = V_{1} + V_{2} + V_{3} + ...$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ 600 = x \times 200$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ x = 3$
So, there should be 3 capacitors in series to obtain the required potential difference.
The equivalent capacitance of the 3 capacitors in series is
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{1}{C_{eq}} = \frac{1}{6} + \frac{1}{6} + \frac{1}{6}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C_{eq} = 2$
Now, we require 18$\ \mu F$ capacitance, for this we need 9 such combinations is parallel.
Hence, $9 \times 3 = 27$