Um, I not positive, but I think fish don't need oxygen.
It is the 'crest' part that the green arrow is identifying.
Answer:
intensity because square of the amplitude is proportional to the intensity of the wave
Explanation:
The wavelength of the note is

. Since the speed of the wave is the speed of sound,

, the frequency of the note is

Then, we know that the frequency of a vibrating string is related to the tension T of the string and its length L by

where

is the linear mass density of our string.
Using the value of the tension, T=160 N, and the frequency we just found, we can calculate the length of the string, L: