Final answer:
Coal is the most plentiful fossil fuel in the United States, being the largest domestically produced source of energy and a significant emitter of CO₂. However, the estimated reserves may not be fully reliable and other fossil fuels like petroleum and natural gas also constitute a major part of energy production.
Step-by-step explanation:
The most plentiful fossil fuel in the United States is coal. Coal is not only the largest domestically produced source of energy, but also the nation's biggest contributor to CO₂ emissions when burned. Despite its abundance and its crucial role in U.S. energy production, there is a significant concern with coal as it emits roughly twice as much CO₂ per unit of delivered energy compared to other fossil fuels.
Additionally, the actual reserves of coal can be somewhat uncertain; for example, the United Kingdom had to significantly reduce its estimated coal reserves due to the infeasibility of extracting coal from thin or difficult seams. It's also worth noting, however, that fossil fuels such as petroleum and natural gas collectively supply approximately 80% of the country's primary energy, emphasizing the dominance of fossil fuels in the current energy landscape. The substantial use of these resources is driving the exploration of alternative energy sources to ensure sustainability and energy security in the future.