The wavelength is the distance between one crest/trough to another crest/trough. On the image, it's basically the length between each peak of the wave. You can see that the distance between peaks in wave A are much shorter than the distance between the peaks in wave B.
Thus, wave B has the longer wavelength.
Answer:
https://sensorex.com/ph-calculator/
Explanation:
go to this link and then scroll down a little bit and then you'll find the answer
