The correct answer is D. Capillary water.
Capillary water is the water that is held in the soil by the force of adhesion between the water molecules and the soil particles. It is the water that is readily available to plants for absorption by roots.
Gravitational water is the water that is held in the soil by gravity. It is the water that is found in the large pores of the soil and it is not available to plants for absorption.
Rain water is the water that falls from the sky as rain. It is a mixture of water and other substances, such as dust and pollutants. Rain water can be absorbed by plants, but it is not as readily available as capillary water.
Hygroscopic water is the water that is held to the surface of soil particles by the force of adhesion. It is the water that is found in the very small pores of the soil and it is not available to plants for absorption.