Root pressure becomes insignificant when:
B fits best: Transpiration rate is high. Keep the WATER TRANSPORT basics straight and Transpiration rate is high stands out as the precise answer. Avoid: A) Soil water potential is high; C) At night; D) Humidity is high. 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.