I received a PM asking me to respond.
I agree with the choice most of you have chosen: C.
Once the economic and social usefulness of the motor car was demonstrated and with its superiority to the horse being proved,much of the early hostility to it in rural regions disappeared.
Once A <happened> and B <happened>, <C followed>.
dependent compound clause, independent clause.
So, first, we need that "and" in there. Eliminate D and E. Second, the A and B in the compound clause need to be parallel.
A uses "was demonstrated" so we need something in the same tense. A uses "being proved." Nope. B uses "had been proved." Nope. C uses "proved," which also seems wrong at first glance - but as someone up above said, we don't need to repeat the "was" in this instance. This is an unusual construct (I can't recall seeing something quite like this on any official questions) but we don't need to repeat the "was" because of the parallel structure.
That allows us to narrow to C without dealing with the idiom issue (superiority to / over). I have not seen this particular idiom used on an official question, so I cannot say what the official testwriters would prefer here. Both are used in common English, which leads me to believe that either is fine, but there are cases of other common usages that the testwriters don't like and don't consider correct, so... unless they officially comment on the issue (that is, release a question with an explanation that indicates one usage is wrong), we're kind of stuck on this question.
In general, if you can solve a problem using other hard-and-fast grammar rules, do so. In this case, we know we need the "and" (eliminating D and E) and we know we need parallelism (eliminating A and B), so we can ignore the "superiority" issue altogether.
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Stacey Koprince
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