When plotting instantaneous values of an AC voltage, what shape does the graph form?

The correct shape formed when plotting instantaneous values of an AC voltage is a sine wave. This is due to the fundamental nature of alternating current (AC) electricity, which varies sinusoidally over time. In a sine wave, the voltage levels oscillate smoothly between positive and negative peaks, reflecting the natural behavior of AC as it transitions through its cycles.

The sine wave represents how the voltage changes continuously and periodically, providing a clear visual representation of the AC waveform. This waveform is characterized by its smooth curvature, which contrasts with other shapes. Square waves, for instance, switch abruptly between high and low states without any gradual transition, while triangular waves exhibit linear rises and falls. Pulse waves consist of short bursts of voltage followed by periods of zero voltage, lacking the periodic and smooth characteristics of a sine wave.

Thus, the sine wave effectively embodies the oscillatory nature of AC voltage, making it the correct answer for this question.