What type of wave is least likely to provide easily interpretable output in spectrum analysis?

A complex wave is least likely to provide easily interpretable output in spectrum analysis because it is composed of multiple frequencies and amplitudes, often resulting in a complicated and chaotic waveform. This complexity makes it challenging to discern individual frequency components, as they can overlap and interfere with each other, leading to a spectrum that is difficult to interpret clearly.

In contrast, a sine wave represents a single frequency and has a straightforward, predictable pattern, making its spectrum very easy to analyze. Harmonic waves, which are integral multiples of a fundamental frequency, can also be somewhat predictable, as their components relate mathematically to the fundamental frequency and can be analyzed with relative ease. An impulse wave, while having a sudden change, typically contains a wide range of frequencies, but it is easier to interpret in its time domain representation rather than as a complicated spectrum. Therefore, the inherent complexity of a complex wave leads to challenges in analysis, making it the correct answer in this context.