Answer: Respiratory infections, lung cancer, and heart disease.
Explanation: Most realistic answer choice:
a. Global warming hasn't struck a 'degree' capable of killing millions of people across the globe. Multiple areas are dangerously hot, but most have little to no inhabitants -- otherwise, special weather notices will probably be issued to residents.
b. Not enough damage has been done to the ozone layer; the infamous hole has been recovering for a long period (according to NASA).
c. Acid rain is infrequent and isn't voluminous enough to be deadly over the planet.
"Respiratory infections, lung cancer, and heart disease", on the contrary, influences many populations over the globe in many 'spots' as some ages, current conditions, and more can result in fatal outcomes for those who happen to contract such illnesses from air pollutants, alongside the possibility of these regions already having low AQI and/or thriving industries (etc.)
Hope this helps! God bless.