Shade avoidance response is triggered by decrease in ratio of:
Red:far-red (B) is correct here. This is core PSS-Sec-E-Sensory biology: once you know the definition or pathway step, Red:far-red is the clear fit. The wrong ones are A) Blue:red; C) Green:red; D) UV:red. If the topic is about gradients or potentials, water/solutes move from higher to lower of the relevant quantity.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.
