Question

The vertical displacement of the wave is measured from the ?

Answer 1

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.