D. Pressure vs. temperature
Explanation:
In physical chemistry, this diagram or graph is utilized to predict the conditions like volume, pressure and temperature at which thermodynamic- ally different phases like solid, gases and liquid occur and both states exist at equilibrium.
The simple phase diagrams are the pressure–temperature diagrams of a simple substance like water. In that diagram, x and y axes were correspond to the temperature and pressure. It shows, in the space of pressure and temperature, the equilibrium lines occur between all the 3 phases of water.
