Which main idea does this detail fit? Fossil fuels are composed mostly of hydrocarbons.

This detail provides the main idea that fossil fuels are rich in carbon and hydrogen.

Explanation:

As seen in the question above, fossil fuels are mainly formed from hydrocarbons. Hydrocarbons, in turn, are organic compounds, widely studied because of their wide use, composed exclusively of carbon and hydrogen atoms.

For this reason, when someone claims that "Fossil fuels are composed mostly of hydrocarbons" we can consider the main idea that fossil fuels have a large amount of carbon and hydrogen in their composition, which explains the ability to combust that these fuels have.