Wavelength refers to the distance between two consecutive points in a wave that are in phase, such as two consecutive crests or troughs. It is not the same as the trough itself.
Wave speed refers to how fast a wave travels through a medium. It is not the same as the trough of the wave.
This is not the correct definition of a "trough." The highest point of a wave is called the "crest."
The lowest point of a wave is indeed referred to as the "trough." The trough is the portion of the wave where the displacement of the medium is at its minimum.
The correct statement is D. Lowest point of the wave. The trough is the lowest point in a wave where the displacement of the medium is at its minimum.