Parallel R-C circuit is given in figure bellow
In the stationary regime, we can use complex impedance for capacitor
and R for resistor.
Parallel R-C impedance is
So final expression for parallel R-C circuit impedance
It is obvious that with signal frequency increase, impedance of parallel R-C circuit decreases, since impedance of the capacitor decreases with frequency, and capacitor is in parallel with the resistor.