Answer:
Meter (m)
Step-by-step explanation:
The wavelenght of a light wave is a measure of the distance between two successive crests (or two successive troughs) of a light wave.
Since the SI units for the distance is the meter (m), then the SI unit for the wavelength is also the meter (m).
Wavelength is related to the frequency of the light wave by:
where
c is the speed of light
f is the frequency of the light