The wavelength of light absorbed by P_r form of phytochrome:

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 (\mathrm{P}_{\mathrm{r}}) responses is at about 680 nm. So, the correct answer is '680 nm'