Explanation: To isolate the source of distortion in a receiver's audio output, an oscilloscope is the ideal tool. By visually displaying the AC signal at the output of each audio frequency (AF) amplifier stage, an operator can directly observe the waveform. Distortion manifests as a change in the signal's shape, such as clipping, flattening, or other non-linearities, which would be immediately evident on the oscilloscope screen. Tracing the signal from the input of the AF amplifier chain towards the output, one can pinpoint the exact stage where the waveform first deviates from its expected clean shape, thereby isolating the fault. * **A) Use an oscilloscope to visually display AC signals on the output of each AF amplifier stage:** This is the most direct and effective method. An oscilloscope shows the actual signal waveform, allowing immediate visual detection of any clipping or non-linearity introduced by a faulty stage. * **B) Use a spectrum analyzer to visually display the amplitude and frequency of each AF amplifier stage input:** While a spectrum analyzer can show new frequency components (harmonics) created by distortion, looking at the *input* of stages won't reveal where the distortion is *introduced*. It's also less direct for visualizing waveform integrity than an oscilloscope. * **C) Use a signal generator to inject a 1 KHz signal into the first AF amplifier stage and evaluate audio output:** This helps confirm if distortion exists, but it doesn't isolate the specific stage causing it. You'd still need a tool like an oscilloscope to trace the signal through the stages. * **D) Use a digital voltmeter to measure transistor bias voltages in each AF amplifier stage:** Measuring DC bias voltages checks the transistor's operating point, and incorrect bias *can* cause distortion. However, it's an indirect diagnostic. A DVM won't show the AC waveform's distortion itself, and correct bias doesn't guarantee a distortion-free output under signal conditions.