The correct answer is - Troposphere.
The troposphere is the lowest part of the atmosphere. It is the densest part of it that borders with the lithosphere and the hydrosphere. The troposphere is the part of the atmosphere that supports all the life on Earth.
By polluting the troposphere, the effects on the human health will not be good at all, in fact it can be disastrous. The humans need air for the breathing, and the air that the humans breathe in is from the troposphere, so if the troposphere is polluted, than the humans will breathe in chemicals that are toxic and damaging for the human body. That will result in big problems with the health among the humans, especially health problems with the respiratory system, the heart, and cancer.