Explanation: A square-law vacuum tube detector operates in a non-linear region where the output current is proportional to the square of the input radio frequency (RF) voltage. This fundamental characteristic leads to several performance traits: A) **High sensitivity is provided due to the audio output signal amplitude being proportioned to the square of the radio frequency voltage.** For weak Amplitude Modulated (AM) signals, the squaring action of the detector allows it to produce a discernible audio output when a linear detector might not. The detected audio component is indeed proportional to the square of the input RF carrier amplitude and the modulation index, making it sensitive to weak signals. B) **High percentage of second harmonic distortion.** Since the output is proportional to the square of the input, any input signal at frequency 'f' will produce a component at '2f' (the second harmonic). When demodulating AM, this applies to the modulating audio signal, causing second harmonic distortion in the recovered audio. C) **Works on weak signals only.** Square-law detectors are most effective and yield their best performance for weak signals. For stronger signals, they tend to introduce more distortion and become less efficient than linear envelope detectors, which are typically used for robust AM signals. As all three statements accurately describe characteristics of a square-law type of vacuum tube detector, "D) All of the above" is the correct choice.