Explanation: Class A audio amplifiers operate such that the active device (transistor or vacuum tube) conducts for the entire 360 degrees of the input signal cycle. This continuous conduction ensures that the amplifier operates in its most linear region for the full waveform, producing an output signal that is a highly faithful and amplified reproduction of the input. This linearity inherently results in very low harmonic distortion of the output signal, making "low distortion of the output signal" the chief advantage for high-fidelity audio applications compared to other classes (like B or AB) which conduct for less than the full cycle, introducing more non-linearity and distortion (e.g., crossover distortion). Options A, C, and D are incorrect. Hum currents (A) are related to power supply design, not the inherent operating class. Increased regeneration (C) is generally undesirable in amplifiers as it can lead to instability or oscillation, not an advantage. The need for cathode bypass capacitors (D) depends on the specific circuit design and gain requirements, not a defining advantage of Class A operation regarding signal quality.