Final answer:
A trough in wave terminology refers to the lowest point of a wave cycle, as opposed to the crest, which is the highest point.
Step-by-step explanation:
In the context of waves, a trough is the lowest point the medium sinks to in a wave cycle. This is in contrast to the crest, which is the highest point the medium rises to. The amplitude of a wave is measured from the midpoint or equilibrium position to either the crest or the trough.
The wavelength is a separate concept and is defined as the distance between two consecutive identical points on the waveform, typically measured from peak to peak or trough to trough. With this in mind, the correct answer to the question is: b) The lowest point.