Explanation: A spectrum analyzer provides a visual representation of signals in the frequency domain. Its primary function is to display the strength of various frequencies present in a given input signal. The **vertical axis** of a spectrum analyzer is dedicated to showing the **amplitude** (or power level) of the signals detected. This amplitude is typically measured in decibels (dBm, dBV), microvolts, or watts, indicating how strong a particular frequency component is. Conversely, the horizontal axis displays frequency, showing the range of frequencies being analyzed. Options B (Duration) and D (Time) are incorrect because time is generally represented on the horizontal axis of an oscilloscope, which shows amplitude versus time. While some advanced spectrum analyzers can show time-related information (like spectrograms or persistence), the fundamental vertical axis display is amplitude. Option C (Frequency) is incorrect because frequency is displayed on the horizontal axis.