Plant cooling occurs due to 

Transpiration is the evaporation of the water from the stomata of the plants. The water absorbed by the roots is transported up to the leaves where stomata are present. Stomata open during the daytime and increasing temperature results in loss of water in the form of vapours. This is similar to the sweating process that occurs in humans. It cools the plants during high temperature.
Assimilation is the use of minerals by plants.
Guttation is oozing out of water in liquid form during night-time.
Photorespiration is a deviation in photosynthesis when the concentration of oxygen increases.
Transpiration is evaporation of water due to increased temperature during daytime. So, the correct answer is 'Transpiration'.