Guttation is the process of exudation of water droplets from the tips of leaves, usually in the morning. This happens due to root pressure.

Explanation:

Root pressure is the pressure generated within the roots that pushes water upwards. This pressure is caused by active transport of ions into the root cells, which leads to the accumulation of solutes in the root. This accumulation of solutes causes a decrease in water potential in the root, which causes water to move into the root via osmosis. As a result, the water exerts pressure on the root cells, which pushes it upwards.

During the night, when the roots continue to absorb water, the pressure within the xylem vessels may build up to such an extent that it forces out the water droplets from the tips of leaves through hydathodes. This is known as guttation. Guttation occurs in plants that have high rates of transpiration and are actively absorbing water at night.

In contrast, transpiration is the process of loss of water from the leaves in the form of water vapor. It occurs when the stomata on the leaves are open, allowing water to escape through the leaves. Diffusion is the movement of particles from an area of high concentration to an area of low concentration. Osmosis is the movement of water molecules across a semi-permeable membrane from an area of high concentration to an area of low concentration.

Hence, the correct answer is option D - root pressure.