Answer
$C = 2.0~\mu F$
Work Step by Step
We can write the expression for the energy stored in a capacitor:
$U = \frac{CV^2}{2}$
When there are two capacitors, the smallest capacitance occurs when the capacitors are in series, and the largest capacitance occurs when the capacitors are in parallel.
The second smallest energy stored occurs when the capacitor of smaller capacitance is connected individually.
We can find the smaller capacitance:
$\frac{CV^2}{2} = 100~\mu J$
$C = \frac{(2)(100~\mu J)}{V^2}$
$C = \frac{(2)(100~\mu J)}{(10~V)^2}$
$C = 2.0~\mu F$
