If you read the question carefully, you can find the answer right there in it.
In fact, you don't even have to read the whole question ... you only need to read one word.
In fact, you don't even have to read the whole word ... you only need to read part of the word.
The word to read is "wavelength".
The important part of the word is "length".
<em>Now</em> it's time to look at the picture:
#1 is a height.
#2 is a place on the wave
#4 is a place on the wave
<em>#3 </em>is the only thing in the picture that's a "length" of anything.
