The burning of fossil fuels by cars and factories has most affected the cycle.

The correct answer would be the carbon cycle.

Carbon cycle refers to the biogeochemical cycle by which carbon is cycled in an environment via various inter-conversions of carbon compounds.

The burning of fossil fuels by cars and factories release a huge amount of carbon dioxide in the atmosphere.

It is then used by plants for the process of photosynthesis.

Plants products are then consumed by consumers which then release carbon dioxide back into the atmosphere through cellular respiration.

Thus, burning of fossil fuels would affect carbon cycle the most.