Which of the following describes a waveform that consists of half of one distinct waveform and never drops to negative?

A waveform that consists of half of one distinct waveform and never drops to negative is accurately described as a single-phase half-wave rectified waveform. In this context, half-wave rectification means that only one half of the input AC waveform is allowed to pass through while the other half is effectively blocked.

During the positive half of the AC cycle, the output mimics the shape of the input waveform, but when the input signal goes negative, the output waveform is flat at zero. Thus, you see a waveform that consists of only the positive portion and does not appear below the zero voltage line. This results in a pattern where only half of the waveform is present, making it characteristic of single-phase half-wave rectification.

This selective allowance of the waveform describes the half-wave rectification process directly and is what sets it apart from other forms of rectification like full-wave rectification, which utilizes both halves of the AC waveform, or unrectified waveforms, which do not modify the waveform at all.