ARF proteins function as:
The right choice is C: Transcription factors. In Auxin Phytohormone, that matches how the process or concept actually works — the other choices mix up related ideas or use the wrong mechanism. Skip these: A) Receptors; B) Transporters; D) Enzymes. 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.