The correct answer is option 2: capillary water.

Capillary water refers to water that is held in the soil by capillary forces, which are the forces that allow water to move against gravity in narrow spaces, like the small pores between soil particles. This water is readily available to plant roots because it is held in the soil at a tension that allows plants to easily extract it for their growth and development. Capillary water is important for plants as it provides the necessary moisture for their root system.

Option 1, gravitational water, refers to water that drains through the soil under the force of gravity. This water is not readily available to plants as it quickly moves downward into deeper soil layers, beyond the reach of most plant roots.

Option 3, hygroscopic water, refers to water that is held tightly by soil particles and is not available to plants. It is bound too strongly for plants to extract it.

Option 4, bound water, refers to water that is held in the soil in a bound form and is unavailable to plants.

In summary, capillary water is the correct option because it is the water that is readily available to plant roots in the soil.