Step 1: For a series \(RC\) circuit with source amplitude \(V_s\), the capacitor voltage amplitude is \[ |V_C| = V_s \frac{|Z_C|}{|Z_R+Z_C|} = V_s \frac{\tfrac{1}{\omega C}}{\sqrt{R^2+\left(\tfrac{1}{\omega C}\right)^2}}. \]
Step 2: Let \[ x = \frac{1}{\omega C}. \] Then \[ |V_C| = V_s \frac{x}{\sqrt{R^2+x^2}}. \] As \(\omega \to 0\) (i.e., \(x \to \infty\)): \[ |V_C| \to V_s \frac{x}{x\sqrt{1+\tfrac{R^2}{x^2}}} \to V_s. \] For any finite \(\omega > 0\), \(x\) is finite and \(|V_C| < V_s\).
Hence, the maximum amplitude occurs at \[ \omega_0 = 0 \ \text{rad/s (DC)}. \] Final Answer: \(\boxed{0}\).





