Which of the following best explains why air pollution that was once blamed for global cooling is now considered responsible for global warming?...

If you stop and consider what global warming is, the answer choice will come into sharper view.  Global warming is the increase of temperatures world-wide. 

What would cause a world-wide increase in temperatures?  Well, let's think about things that would trap heat, or produce heat and keep it contained.  When you think along the lines produced, trapped, contained heat, answer selection "C" makes a lot of sense: "Scientists mistakenly thought pollution reflected sunlight and heat rather than absorbing it" is the best pick out of the answers listed.  The increase of manufacturing world-wide, the rise in the number of countries consuming electricity, the over-production of gases in the atmosphere, which do that very thing, which trap and absorb heat, is what contributes the most to the phenomenon known as global warming. 

To survey all the answers, answer "A" is "C" in reverse while answer "B" is too general, not specific enough.  Answer selection "D" is generally good but in reverse order of how events occurred or were perceived (global warming occurred and was perceived before global climate change).