Frequency is measured in units of reciprocal time.
Period is measured in units of time.
Phase is a number without units that represents a fraction of a wave.
None of these is measured in meters, so none of them can be the answer.
It must be either amplitude or wavelength.
Amplitude is a quantity that's measured at one or two points in the same wave.
The question is talking about points on consecutive waves.
Wavelength is the only choice left. That must be it.
