Explanation: An oscilloscope is the primary instrument for visually checking the signal quality of an SSB transmission. It displays the waveform of the signal (voltage against time), allowing an operator to observe the modulation envelope. For SSB, quality issues often manifest as non-linearity, such as clipping or "flat-topping" of the peaks due to overdriving the transmitter or excessive audio gain. The oscilloscope clearly shows these distortions, indicating poor linearity and potential for generating unwanted splatter or harmonics. A field strength meter (A) only measures the signal's strength, not its waveform or quality. A signal level meter (B) provides a numerical value for amplitude but cannot reveal distortion. A sidetone monitor (C) is an audio feedback for Morse code (CW) transmissions, entirely unrelated to SSB signal quality analysis.