Answer
$C = 4.0\times 10^{-5}~F$
Work Step by Step
On the graph, we can see that $X_C = 100~\Omega$ when $\omega_d = 250~rad/s$
We can find $C$:
$X_C = \frac{1}{\omega_d~C}$
$C = \frac{1}{\omega_d~X_C}$
$C = \frac{1}{(250~rad/s)(100~\Omega)}$
$C = 4.0\times 10^{-5}~F$
