The distance shown in the diagram is most likely referring to the distance between two similar points on a wave. Depending on what points the distance measures, the correct answer could vary:
- If the distance measures from one crest to the next crest, it is called the wavelength.
- If it measures from one trough to the next trough, it is still called the wavelength.
If the distance is measuring the height of a wave from the equilibrium position to the crest, that would be called the amplitude.
Since the question indicates "distance," the most correct answer is likely:
D. wavelength.
If you are referring to a specific label in the diagram, please specify which points are measured for a more precise identification.