Explanation: An oscilloscope fundamentally displays electrical signals in the **time domain**, showing amplitude (vertical axis) versus time (horizontal axis). This allows you to visualize waveform shapes, periods, and phase relationships, observing how a signal changes over a period. In contrast, a spectrum analyzer displays electrical signals in the **frequency domain**. It shows amplitude or power (vertical axis) versus frequency (horizontal axis). This reveals the distribution of power across different frequency components present in a signal, making it ideal for analyzing bandwidth, harmonics, modulation characteristics, and spurious emissions in radio frequency signals. Therefore, option C correctly identifies this critical distinction between the two instruments. Option B incorrectly reverses these roles. Option A is too vague, as both instruments perform both display and measurement functions. Option D falsely limits their applications; both can operate across a wide range of frequencies, including RF.