Explanation: A coupling capacitor is fundamentally designed to block the steady flow of direct current (DC) while allowing alternating current (AC) signals to pass through. Capacitors consist of two conductive plates separated by a dielectric, which is an electrical insulator. When DC voltage is applied, charge accumulates on the plates, and once fully charged, the capacitor acts as an open circuit, effectively blocking the DC. However, with AC, the voltage constantly changes polarity. This causes the capacitor to repeatedly charge and discharge, allowing the AC signal energy to be transferred from one part of a circuit to another. This is crucial for isolating DC bias voltages between amplifier stages while still transmitting the desired radio frequency (RF) or audio frequency (AF) signals. Therefore, option A is correct. Option B is incorrect because capacitors pass AC, they don't block it. Options C and D are incorrect because while capacitors are components in resonant circuits, a *coupling* capacitor's primary role is signal transfer and DC isolation, not specifically to set or alter the resonant frequency of a circuit.