Answer
Natural gas
Work Step by Step
The costs are:
Electric heaters:$\frac{0.12\frac{dollars}{kWh}*\frac{1kWh}{3600kJ}}{1}=3.33*10^-5\frac{dollars}{kJ}$
Gas heater: $\frac{1.24\frac{dollars}{therm}*\frac{1therm}{105500kJ}}{0.87}=1.35*10^-5\frac{dollars}{kJ}$
Oil heater:$\frac{2.3\frac{dollars}{gal}*\frac{gal}{138500}}{0.87}=1.91*10^-5\frac{dollars}{kJ}$
The lowest cost energy source for heaters is natural gas.
