Answer: c. burning of fossil fuels.
Step-by-step explanation:
Carbon Dioxide and other greenhouse gases like it are the reason some of the sun's heat is trapped in the atmosphere instead of leaving back into space. Any process which increase the amount of carbon dioxide in the atmosphere therefore can increase global warming.
One of the major human activities that involves releasing carbon dioxide is the burning of fossil fuels. We use fossil fuels for many things including producing electricity and driving. When this happens we release a lot of carbon dioxide into the atmosphere thereby making global warming worse.