The correct answer is option 4, Transpiration. Transpiration is the process by which plants lose water through the small pores on the surface of their leaves, called stomata. When there is excessive transpiration, which can occur due to factors like high temperature, low humidity, or lack of water availability in the soil, plants may not be able to absorb enough water to compensate for the loss. As a result, the cells in the leaves and stems lose turgidity, causing the plant to wilt.

Option 1, Respiration, is not correct because while respiration is a vital process in plants, it is not responsible for wilting.

Option 2, Guttation, is the exudation of water droplets from the tips or edges of leaves, which occurs due to root pressure. It is not the main cause of wilting.

Option 3, Absorption, is the process by which roots take up water and nutrients from the soil. While lack of absorption can contribute to wilting, it is not the direct cause.

Overall, excessive transpiration is the main reason for plant wilting.