Q. Which of the following phytohormone is responsible for regulating the opening and closing of stomata ?

A) GA
B) ABA
C) IBA
D) kinetin

... Answer is B). ABA

Abscisic acid (also called ABA), was discovered and researched under two different names before its chemical properties were fully known, it was called dormin and abscicin II. Once it was determined that the two compounds are the same, it was named abscisic acid. The name "abscisic acid" was given beceuse it was found in high concentrations in newly abscissed or freshly fallen leaves

In plants under water stress, ABA plays a role in closing the stomata. Soon after plants are water-stressed and the roots are deficient in water, a signal moves up to the leaves, causing the formation of ABA precursors there, which then move to the roots. The roots then release ABA, which is translocated to the foliage through the vascular system and modulates the potassium and sodium uptake within the guard cells, which then lose turgidity, closing the stomata.