The correct answer is C) frequency. The number of cycles per second occurring in AC voltage is known as the frequency. Frequency is a measure of the number of complete cycles that occur in a given time period, typically one second. This is a fundamental characteristic of alternating current (AC) voltage and is measured in Hertz (Hz). The other options are incorrect because: A) Phase angle refers to the angular relationship between the voltage and current waveforms in an AC circuit. B) Half mode is not a recognized term in this context. D) Waveform describes the shape of the AC voltage or current over time, not the number of cycles per second.