The correct reason why the unsaturated zone is not filled with water is:
It is permeable and water flows through it.
The unsaturated zone, also known as the vadose zone, is the area of soil above the water table where the soil pores contain both air and water. Although it has water, it is not fully saturated, and water can move through this zone due to gravity and capillary action. The presence of air in the pores allows for this permeability, distinguishing it from the saturated zone, which is filled with water.