Explanation: Parasitic oscillations are unwanted self-sustained oscillations within an amplifier circuit, occurring at frequencies other than the intended operating frequency. These unwanted oscillations draw power from the amplifier's supply, reducing the energy available for the desired signal. This directly leads to **reduced efficiency of the amplifier** (B), as useful power is diverted. The high-frequency or low-frequency voltage swings associated with parasitic oscillations can cause the active device (tube grid, transistor base) to draw unintended current. This shifts the device's operating point, resulting in a **change of bias** (A). An altered bias can further contribute to non-linear operation. Finally, these parasitic signals are superimposed onto the desired signal. If the amplifier is processing a modulated wave, the parasitic signals will interfere with the intended modulation, causing the output waveform to deviate from its correct shape, leading to **distortion of the modulated wave** (C). Because parasitic oscillations cause all these detrimental effects, "All of the above" is the correct answer.