Before answering the question, first we have to understand a longitudinal wave.
A longitudinal wave is a type of mechanical wave in which the direction of wave propagation is parallel to the particle vibration of the medium.
In this type of wave, there will be compressions and rarefactions. Compressions are the high pressure regions where the particles of the medium are very close to each other. The rarefactions are the low pressure regions of a longitudinal wave where the particles are not so close to each other.
Hence, a longitudinal wave is a series of compressions and rarefactions.
The wavelength of a longitudinal wave is defined as the distance between two successive compressions or rarefactions.
Hence, the correct answer to the question is C) by measuring the distance between adjacent rarefactions.
