Explanation: Determining power in an AC circuit can be achieved through two primary methods. Firstly, a **wattmeter** (Answer A) is a dedicated instrument designed to directly measure real power (in watts). It senses both the voltage across the circuit and the current flowing through it, and inherently accounts for the phase relationship between them, which is crucial in AC circuits. This provides a practical, direct measurement. Secondly, power can be **calculated** (Answer B) using the formula P = V x I x PF. Here, P represents the real power, V is the RMS voltage, I is the RMS current, and PF is the power factor. The power factor (cosine of the phase angle between voltage and current) is essential in AC circuits with reactive components (like inductors and capacitors commonly found in radio frequency circuits) because it accounts for the portion of apparent power that actually does work. Since both direct measurement with a wattmeter and calculation using the voltage, current, and power factor are valid and commonly used methods, **C is the correct answer**, as it comprehensively includes both approaches.