(this is earth science but there is no option for that)which of the following would be most impacted by a decrease in the use of fossil fuels?

water cycle

volcanic activity

carbon cycle

nitrogen-fixing bacteria

Decrease in burning of fossil fuels will affect carbon cycle the most.

Explanation:

Carbon compounds circulate within the biosphere as part of carbon cycle.  In terrestrial ecosystems Plants acquire carbon dioxide through photosynthesis and in aquatic ecosystems plankton absorb carbon dioxide.  

Dead plants bodies deposited underground for a large number of years get converted to fossil fuels and the burning of these fossil fuels releases carbon dioxide into the atmosphere.

Carbon dioxide is also released into the atmosphere through processes like respiration and decomposition of dead organic matter. If burning of fossil fuels is decreased it will be the carbon cycle which will be affected the most.