(b) transpiration and temperature regulation.
Loss of water vapour from aerial parts of plants is transpiration, which helps in temperature regulation of the plant.
Transpiration is specifically the loss of water in vapour form from aerial parts (leaves, stems) through stomata. The textbook notes that plants lose large amounts of water through stomata; this evaporative loss cools the plant surface, regulating its temperature. Option (c) and (d) describe translocation, not transpiration. Option (a) is a distractor — photosynthesis requires water but transpiration does not directly "help" photosynthesis.