Explanation:

Only statements 1&2 are correct.

• Statement 3rd is incorrect: Air density decreases with increasing height during an inversion. This causes a density stratification of the air, with the coolest, densest or heaviest air near the surface, and the density decreasing with increasing height. As a result, air can move only horizontally within the inversion. 

• Temperature inversions occur when a layer of warm air traps cool air near the Earth’s surface. This is the inverse of what normally happens.

• Most of the time, for example, the Earth’s surface is warm during the day and air gets colder the higher you go.

• A temperature inversion usually happens during the night when the ground temperature drops because it’s no longer heated by the sun.

Why in news?

• Mumbai’s air quality has worsened than the National Capital's quality of air. The phenomenon called “air inversion” or temperature inversion is behind the situation.

Hence option 2nd is correct.