Explanation: A two-tone test is a crucial diagnostic for Single Sideband (SSB) transmitters. It involves feeding two pure audio tones of equal amplitude and slightly different frequencies (e.g., 700 Hz and 1900 Hz) into the transmitter's microphone input. An ideal, perfectly linear SSB transmitter should only produce two corresponding RF output tones – the two desired SSB products. If the transmitter's amplifier stages are non-linear, they will generate additional, unwanted intermodulation distortion (IMD) products. These IMD products appear as new frequencies in the RF output, typically at odd multiples of the difference between the input tones (e.g., 2f1-f2, 2f2-f1). On an oscilloscope, these unwanted products manifest as "shoulders" or "splatter" around the two desired output tones, indicating non-linearity and potential out-of-band emissions. Therefore, the test directly illustrates the linearity of the SSB transmitter's amplification chain. Options B and C are incorrect because a two-tone test for SSB does not evaluate carrier phase shift or frequency modulation; SSB is a form of amplitude modulation. Option D, sideband suppression, is a different characteristic (the ratio of desired to undesired sideband power), typically measured with a single tone. The two-tone test specifically assesses *new*, unwanted intermodulation products caused by non-linearity, not just the original unwanted sideband.