The correct answer is "b."
The wavelength of a wave is the distance between two consecutive crests. This applies to a transverse wave, where the wavelength can always be determined by measuring the distance from crest to crest or from trough to trough. However a longitudinal wave has no crest or trough. How can the wavelength be determined ? In this case, the wavelength can be determined by measuring the distance from one compression to the next compression or from a rarefaction to the next rarefaction.