The point on the wave the student should mark as the end of the wavelength is the next compression. (Option C).
What is wavelength of a longitudinal wave?
In a longitudinal wave, a wavelength is the distance between two successive compressions or rarefactions.
Compressions are regions where the particles are closer together, and rarefactions are regions where the particles are spread apart.
Given that the student marks the beginning of the wavelength at a compression, the end of the wavelength should be at the next compression.
Thus, we can conclude that the correct answer is the next compression.