The correct answer is C. Using fossil fuels for energy
Step-by-step explanation:
Carbon dioxide is a type of gas composed by two atoms of oxygen and one atom or carbon, this gas is found on the atmosphere and produced through natural processes that include volcanic eruptions, plant and animals respiration, decomposition and even fires caused naturally. However, in the last decades, the amount of carbon dioxide in the atmosphere has increased unexpectedly and contributed to other problems such as global warming and the greenhouse effect.
This issue has been caused mainly by the human activity that includes the burning of fossil fuels ( oil, gas, and coil) extensively used for energy and transportation as well as the industries that produces important amounts of carbon dioxide as part of manufacturing processes. Considering this, the contributor that is caused solely by human activity is "using fossil fuels for energy" as fossil fuels are only used for human beings to produce energy and it is not part of natural forces, but related to human activities such as the gasoline used transportation or the gas used for cooking.