Explanation: A power detector, often employing a diode, can operate in different modes depending on the input signal's strength. At very low input power levels, the diode operates in its **square law region**. In this non-linear range, the diode's output current (and thus voltage) is approximately proportional to the square of the input voltage, effectively detecting the input power. As the input power increases and becomes strong enough to fully forward-bias the diode during the signal peaks, the detector transitions into its **linear region**. Here, the diode acts more like a peak detector, where the output voltage becomes directly proportional to the peak amplitude of the input voltage. Therefore, a power detector can function as either a square law detector (for weak signals) or a linear detector (for stronger signals), making 'C' the correct answer. Options 'A' and 'B' are too restrictive as they imply only one mode of operation.