The Pfr form of phytochrome absorbs light maximally at:
C fits best: 730 nm. Keep the PHOTOPERIODISM basics straight and 730 nm stands out as the precise answer. Avoid: A) 660 nm; B) 680 nm; D) 480 nm. Light and flowering items often hinge on which photoreceptor or day-length rule is in play.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.