Final answer:
Decomposition is a natural source of carbon emissions, while burning fossil fuels and deforestation are anthropogenic sources that contribute to global warming. Photosynthesis, on the other hand, is a process that absorbs CO2 and is not a source of emissions.
Step-by-step explanation:
The natural source of carbon emissions among the options provided is decomposition. During this process, bacteria and fungi break down dead organic matter, releasing carbon dioxide (CO2) and methane (CH4) into the atmosphere. This is part of the Earth's carbon cycle, which is a natural process that has been occurring long before human influence. Contrastingly, the burning of fossil fuels, deforestation, and photosynthesis have either been significantly altered or are processes influenced by human activities. Burning fossil fuels and deforestation are major anthropogenic sources of carbon emissions, significantly enhancing the greenhouse effect and contributing to global warming. Photosynthesis, however, is a process where plants absorb carbon dioxide and is not a source of emissions but rather a natural carbon sink.