Explanation: Testing the amplitude linearity of a single-sideband (SSB) transmitter is crucial to prevent intermodulation distortion (IMD) and "splatter," which causes interference on adjacent frequencies. A **two-tone audio-frequency sine wave** is the ideal input for this test. When two pure, non-harmonically related audio sine waves are fed into an ideal, linear SSB transmitter, the output will consist of only two corresponding radio frequency (RF) tones. When viewed on an oscilloscope, the envelope of this two-tone RF signal forms a characteristic, predictable pattern (often described as a "bow-tie" or "Christmas tree"). Any deviation from this ideal pattern, such as flattened peaks or other distortions, clearly indicates non-linearity in the transmitter and the generation of unwanted intermodulation products. A single audio frequency sine wave produces a constant-amplitude RF output, which offers no envelope variation to assess linearity or intermodulation distortion. Whistling in the microphone provides an uncontrolled, complex, and irreproducible input. An audio frequency square wave is composed of a fundamental and many odd harmonics, generating a very complex output signal that is difficult to interpret for linearity compared to the precise two-tone test.