Exposure to far-red light reverses the effect of:
B fits best: Red light. Keep the PHOTOPERIODISM basics straight and Red light stands out as the precise answer. Avoid: A) Blue light; C) UV light; D) Green light. 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.