(d) The capacitance across $A$ and $B$
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$= \frac{C_{1}}{2} + C_{1} + C_{1} = \frac{5}{2}C_{1}$
As $Q = CV,$
$1.5\mu C = \frac{5}{2}C_{1} \times 6$
$\Rightarrow C_{1} = \frac{1.5}{15} \times 10^{- 6} = 0.1 \times 10^{- 6}F = 0.1\mu F$