Answer: Natural gas.
Explanation: In United states, around 7.5% of electricity is generated using hydro power. 27% of electricity is generated using coal. 35% electricity is generated using natural gas and 1.5% is generated using biomass.
So, comparing the above data, the maximum percentage (35%) of electricity is generated using natural gas.
Hence, About half of the electricity in the United States is generated using natural gas.