Is the coldest layer of the atmosphere.

The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −100 °C (−148.0 °F; 173.1 K).  your welcome lols

The thermosphere. When the mesosphere extends past the stratosphere the temperature decreases. In the thermosphere the air is thin so, there are far less air molocules. Temperatures in there can get as low as -90 degrees Celcius. 

So, The thermosphere is the coldest layer of the atmosphere

Answer b is the answet