Answer:
Coal.
Step-by-step explanation:
Steam power plant has the lowest efficiency in electricity production which can only be run with the use of fossil fuels such as coal. The efficiency of this power plant is the lowest because large amount of coal is converted into smoke and other gases and very less amount of coal into heat energy. This coal is used to produce heat energy which can be used in the heating of water to produce steam. This steam is used to move turbines which turn mechanical energy into electric energy.