Answer: d.an increase in the water vapor content of earth's atmosphere
Step-by-step explanation:
Fossil fuels are fuels formed in the earth from plant or animal remains. It is mostly composed of coal, natural gas and petroleum. Combustion of fossil fuels also produces pollutants such as carbon dioxide, nitrogen dioxide and others.
Fuel cells are electrochemical cells that converts the chemical energy of a fuel and an oxidizing agent into electricity by use of redox reactions. They mostly involve use of hydrogen and oxygen which forms water as a bye product.


Thus if all cars switched from burning fossil fuels to fuel cells, an increase in the water vapor content of earth's atmosphere.