Explanation: A spectrum analyzer's fundamental purpose is to display the distribution of signal power as a function of frequency. Therefore, the horizontal axis (x-axis) represents the range of frequencies being analyzed. Operators set the start and stop frequencies, or a center frequency and span, to observe the spectral content within that band. Options A (Amplitude) and B (Voltage) are incorrect because these quantities, representing the strength or power level of the signals, are displayed on the vertical axis (y-axis) of a spectrum analyzer. Option C (Resonance) is a characteristic of a circuit or antenna at a specific frequency, not a direct axis display. While a spectrum analyzer can help identify a resonant frequency by showing a peak in power, the axis itself still displays frequency.