Answer
b) coal
Work Step by Step
According to Figure 34.8 on page 407, these are fuels used for electricty generation in the United States in 2012 by percentage:
Coal: 40%
Natural gas: 28%
Nuclear fuel: 20%
Hydroelectric dams: 7%
Other renewable energy sources: 4%
Oil: 1%
Out of these, coal is clearly the largest category. Therefore, the major source of energy in the United States is coal.