The correct answer is (a), Ionosphere.
The homosphere is the lower part of the Earth’s atmosphere, where the composition of the air is relatively constant. It extends from the surface of the Earth to an altitude of about 100 kilometers. The ionosphere is the region of the Earth’s atmosphere that is ionized by solar radiation. It extends from an altitude of about 100 kilometers to an altitude of about 600 kilometers. The mesosphere is the layer of the Earth’s atmosphere that is located between the stratosphere and the thermosphere. It extends from an altitude of about 50 kilometers to an altitude of about 85 kilometers. The stratosphere is the layer of the Earth’s atmosphere that is located between the troposphere and the mesosphere. It extends from an altitude of about 10 kilometers to an altitude of about 50 kilometers.
The ionosphere is not included in the homosphere because the composition of the air in the ionosphere is not constant. The air in the ionosphere is ionized by solar radiation, which means that the atoms and molecules in the air have lost or gained electrons. This ionization causes the air in the ionosphere to have different properties than the air in the homosphere.