Why are a plant's stomata generally open during the day and closed at night?

 Stoma (Stomata-plural)  controls how much carbon dioxide is coming into the leaf and, more importantly, how much water is leaving the leaf. 
During the day, specially when it is very hot and dry and these plants would lose too much water through evaporation,  the stomata remain closed. Once it is daylight and the stomata close, they convert  acid back into carbon dioxide that then is available for photosynthesis. At nighttime, the stomata  open. The plants bring in carbon dioxide at night. 

The awnser is b i believe