Question

the main way that humans have disrupted the carbon cycle is by...
the cultivation of food and animal products
disrupting the carbon dioxide exchange of the worlds major oceans
the burning of tropical and subtropical forests
the combustion of fossil fuels

Explanation:

The combustion of fossil fuels releases large - amounts of carbon dioxide that was previously sequestered in the Earth's crust, significantly increasing the amount of carbon in the atmosphere and disrupting the natural carbon cycle. While other options have an impact, fossil - fuel combustion is the main contributor to carbon - cycle disruption on a global scale.

Answer:

The combustion of fossil fuels