In short day plants, flowering occurs when there is:
The right choice is B: Low or absence of Pfr. In PHOTOPERIODISM, that matches how the process or concept actually works — the other choices mix up related ideas or use the wrong mechanism. Skip these: A) High Pfr; C) Continuous red light; D) High Pr accumulation. A quick check: if an option needs energy, pumps, or the opposite direction of movement, ask whether that really applies.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.