The correct answer is C. two phase system with soil and water.
A fully saturated soil is a soil that contains as much water as it can hold. In this state, the soil pores are filled with water and there is no air present. This can happen when the soil is flooded or when it rains heavily.
A two phase system is a system that contains two phases, or states of matter. In the case of a fully saturated soil, the two phases are soil and water. The soil particles are the solid phase, and the water is the liquid phase.
The other options are incorrect. Option A is incorrect because a fully saturated soil is not a one phase system. It is a two phase system. Option B is incorrect because a fully saturated soil does not contain air. Option D is incorrect because a fully saturated soil is not a three phase system. It is a two phase system.