The whole question is talking about the amplitude of a wave that's transverse and wiggling vertically.
Equilibrium to the crest . . . that's the amplitude.
Crest to trough . . . that's double the amplitude.
Trough to trough . . . How did that get in here ? Yes, that's the wavelength, but it has nothing to do with vertical displacement.
Frequency . . . that's how many complete waves pass a mark on the ground every second. Doesn't belong here.
Notice that this has to be a transverse wave. If it's a longitudinal wave, like sound or a slinky, then it may not have any displacement at all across the direction it's moving.
It also has to be a vertically 'polarized' wave. If it's wiggling across the direction it's traveling BUT it's wiggling side-to-side, then it has no vertical displacement. It still has an amplitude, but the amplitude is all horizontal.
