The electric water heater may be more modern than the gas water heater

tough motherducker...undoubtedly..

meaning: EWH more modern than GWH
using E almost = 3X as using G.

now...parallelism is an issue here...
we have USING then correct verb costs, then we have a contrast idiom as much as. When using such an idiom, we need to have it in X as much as Y form. since we have X in a noun form then verb, the same must be in Y part.

A. than the gas water heater, but those homeowners who understand the value of a dollar are aware that using electricity to heat water costs almost three times as much as it does to use gas.
parallelism error...

B. than the gas water heater, but those homeowners who understand the value of a dollar are aware that using electricity to heat water costs almost three times as much as using gas.
this one has a very subtle error...
all good except for the last part...2 meanings can be understood...
Using E to heat costs as much as using gas does
Using E to heat water costs as much as using gas..?!?!?

C. than the gas water heater, but those homeowners who understand the value of a dollar are aware that to use electricity to heat water costs almost three times as much as to use gas.
to use costs - kind of awkward...+ missing of the verb in Y part might lead to ambiguities..

D. than the gas water heater, but those homeowners who understand the value of a dollar are aware that using electricity to heat water costs almost three times as much as using gas does.
this one seems to rectify the error in B...this one is a more stronger answer choice than B and C!

E. than the gas water heater, but those homeowners who understand the value of a dollar are aware that using electricity costs almost three times as much as using gas to heat water.
using electricity costs almost three times...wait! where is to heat water? now the sentence implies that in general, using electricity costs more...out!

D is so far the best answer.