Final answer:
The burning of fossil fuels is the activity that increases the amount of carbon in the atmosphere, contributing to global warming.
Step-by-step explanation:
Among the activities listed, the one that increases the amount of carbon in the atmosphere is the burning of fossil fuels. While animals eating plants and the process of fossilization are both part of the natural carbon cycle, they do not lead to a net increase in atmospheric carbon.
Photosynthesis, in fact, reduces atmospheric carbon by incorporating it into plant biomass.
The burning of fossil fuels, however, releases stored carbon as carbon dioxide gas, contributing significantly to the rise in atmospheric carbon levels and thus to global warming and climate change.