Answer
We can rank the three curves according to the corresponding value of $R$:
$c \gt b \gt a$
Work Step by Step
We can write an expression for the impedance of an RC circuit:
$Z = \sqrt{R^2+(\frac{1}{\omega_d~C})^2}$
We can see that as $\omega_d$ increases, the value of $(\frac{1}{\omega_d~C})^2$ decreases and $Z$ approaches the value of $R$
We can rank the three curves according to the corresponding value of $R$:
$c \gt b \gt a$
