B, nitrogen is the main component of air, using liquid nitrogen will not pollute the environment, so the option statement is wrong.
C, liquid nitrogen has a very low boiling point, vaporizes when spraying, and is insoluble in water, so the option is incorrectly stated.
D, the use of liquid nitrogen as a rain enhancement agent is mainly due to the heat absorption when liquid nitrogen is gasified, and the ambient temperature is lowered, so that the water vapor in the air condenses into water droplets and falls without producing other substances, which is a physical change, so the option statement is correct.
Therefore, choose: d.