The wavelength of light absorbed by Pr form of phytochrome is _____

The correct option is B 680 nm
Light acts as a signal to initiate and regulate photoperiodism and photomorphogenesis. There are two light-sensing systems involved in these responses, the blue light-sensitive system and the red light-sensitive or phytochrome system. The action spectrum of the light needed for P${}_r$ responses is at about $680$ nm.

So, the correct answer is '680 nm'