this layer of earth’s atmosphere has the highest density of any atmospheric layer and contains most of the water vapor in the atmosphere.

The troposphere

Explanation:

The troposphere holds nearly all of Earth’s water vapor. It’s where most clouds ride the winds and where weather occurs. Water vapor and air constantly circulate in turbulent convection currents. Not surprisingly, the troposphere also is by far the densest layer.

b. niger

explanation:

81 is a composite number.