which fossil fuel is the most widely used in the United States

I believe it is coal because it could be used for electric energy. There are a lot of different factors.

When considering which fossil fuel would be most widely used in the United States, this would signify that which would be most known, and not only this, but also which would be more consumable. So, when considering this, we would consider first the plant, and then the type of fuel this would contain. This would be known as Steam, and also the fuel would be considered to be RFO/Coal/Natural Gas and which this would actually be fossil fuel is the most widely used in the United States