Explanation

(b) Capacitive reactance is given by
$X_{C} = \frac{1}{\omega\ C}$
Where C is capacitance and $\omega$ the angular frequency $(\omega = 2\pi f).$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ X_{C} = \frac{1}{2\pi fC}$
$\ \Rightarrow \ \ \ \ \ \ \ \ \ \ \ X_{C} \propto \frac{1}{f}$ $\ $
Hence, when frequency $f$ increases capacitive reactance decreases.