The correct answer is option C: Fundamental.

Explanation:
When discussing signals and waveforms, it is important to understand the concept of harmonics. Harmonics are multiples of the fundamental frequency of a signal. The fundamental frequency is the lowest frequency component of a complex waveform.

In the context of this question, we are given a signal and asked about its second harmonic. The second harmonic is the frequency component that is twice the frequency of the fundamental frequency. In other words, if the fundamental frequency of the given signal is f, then the second harmonic would be 2f.

To understand why the second harmonic is twice the fundamental frequency, we need to consider the nature of harmonics. When a complex waveform is analyzed using Fourier analysis, it can be decomposed into a series of sine waves of different frequencies, amplitudes, and phases. The fundamental frequency represents the first harmonic, and subsequent harmonics are integer multiples of the fundamental frequency.

In terms of the Fourier series representation, the fundamental frequency corresponds to the first term in the series, and the second harmonic corresponds to the second term in the series. The second term has a frequency that is twice the frequency of the first term, hence the second harmonic is twice the fundamental frequency.

To summarize, the second harmonic of a given signal is twice the frequency of the fundamental frequency because it represents the second term in the Fourier series representation of the signal. The fundamental frequency is the lowest frequency component of a complex waveform, and subsequent harmonics are integer multiples of the fundamental frequency.