Increase in molecular size affects diffusion rate by:
C fits best: Decreasing it. Keep the BASICS of PLANT basics straight and Decreasing it stands out as the precise answer. Avoid: A) Increasing it; B) Not affecting it; D) Stopping it completely. 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.