a. it limits water loss through transpiration
The stomata are small openings in epidermis (found on the leaf) that allow carbon dioxide, water, and oxygen to enter and exit the leaf. Transpiration is water movement, including evaporation, in a plant. So just think about it. If it's a hot, sunny day, wouldn't it make sense for the plant to keep its stomata closed so that it doesn't lose water through evaporation? The cacti would close it during the day and leave it open at night since the sun isn't out, and therefore wouldn't lose as much water. Hope this helped :)
