HY5 protein acts as a:
Correct option is C, Positive regulator of photomorphogenesis. In PHYTOCHROME problems like this, the accurate statement is Positive regulator of photomorphogenesis; the others are common mix-ups. Not these: A) Negative regulator; B) Photoreceptor; D) Enzyme. Tissue transport questions usually turn on xylem vs phloem, living vs dead cells, or source vs sink.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.