Abscisic acid $(ABA)$ is an antitranspirant. It prevents transpiration by:

• A
  Forming a film impervious to water over the stomata
• B
  Preventing $K^+$ ion uptake,thus decreasing the osmotic concentration of the guard cells and keeping them flaccid
• C
  Inducing starch formation,thus decreasing the osmotic concentration of the guard cells and keeping them flaccid
• D
  Increasing respiration,thus increasing $CO_2$ concentration in the guard cells which helps in keeping the guard cells flaccid