An ac source is connected to a capacitor C . Due to decrease in its operating frequency:

Capacitive reactance is given by,

$$\begin{gathered} X_C=\frac{1}{\omega C} \\ \Rightarrow X_C\propto \frac{1}{\omega } \end{gathered}$$

Clearly, as $\omega$ decreases the value of the $X_C$ will increase.

Now, as impedance of the circuit is decreasing and as we know, conduction current will be equal to the displacement current, hence displacement current will decrease.