The correct answer is: stratosphere.
This is because their temperatures are rising instead of decreasing.
There are 4 primary layers of the atmosphere on Earth:
Troposphere (6-20km), the layer that we live in, where the weather occurs. Only nitrogen and oxygen present.
Stratosphere (20-50 km), where the airplanes fly, contains the ozone layer, which absorbs harmful radiation from the sun.
Mesosphere (50-85 km), the coldest region of Earth’s atmosphere (-100 C), protective layer
Thermosphere (80-690), the hottest (1500 C) and the thickest layer which consists of the ionosphere and the exosphere.