Answer
Natural gas
Work Step by Step
The costs are:
Natural gas: $\frac{0.012\frac{dollars}{ft^3}*\frac{1ft^3}{1025Btu}}{0.85}=1.38*10^-5\frac{dollars}{Btu}$
Heating oil:$\frac{2.2\frac{dollars}{gal}*\frac{gal}{138700Btu}}{0.75}=2.11*10^-5\frac{dollars}{Btu}$
Electric heaters:$\frac{0.11\frac{dollars}{kWh}*\frac{1kWh}{3412Btu}}{0.90}=3.58*10^-5\frac{dollars}{Btu}$
The lowest cost energy source for water heaters is natural gas
