What happens to the density of air with increase in height? 

Decreases 

The correct answer is option 2. Decreases.

The density of air decreases with an increase in height.

As you move further away from the Earth's surface, the air becomes "thinner," meaning there are fewer air molecules per unit volume. This results in a lower density.

Here's why:

Gravity: Earth's gravity pulls air molecules towards the surface, resulting in a higher concentration of molecules at lower altitudes.

Pressure: Air pressure is caused by the weight of the air pressing down. As you move higher, there are fewer molecules above you, leading to a decrease in pressure. Since pressure and density are directly proportional, a decrease in pressure leads to a decrease in density.

Therefore, as you climb higher, the air becomes less dense due to the combined effects of gravity and pressure.