Far-red light converts phytochrome Pfr to:
Go with A — Pr. Under PSS-Sec-E-Sensory biology, this is the standard explanation you’d use in class: it names the real driver or definition, while the rest are nearby but wrong. Not these: B) Pfr; C) Inactive P; D) Chromophore. When two options sound similar, choose the one that matches the textbook definition most tightly.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.