Stomatal opening during light occurs mainly due to:
B fits best: Endosmosis into guard cells. Keep the STOMATA MOVEMENT basics straight and Endosmosis into guard cells stands out as the precise answer. Avoid: A) Exosmosis from guard cells; C) Plasmolysis; D) Loss of K+ ions. 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.