Question

which gas in earths atmosphere has increased over time due to burning fossil fuels?
oxygen
nitrogen
water vapor
carbon dioxide

Explanation:

Burning fossil fuels (like coal, oil, gas) undergoes combustion reactions that release large amounts of carbon dioxide ($CO_2$) into the atmosphere, causing its concentrations to rise over time. Oxygen, nitrogen, and water vapor levels are not significantly increased by this activity.

Answer:

carbon dioxide