I think here it assumes teachers want to teach students in a way that helps them. This however, wasn't stated, but obviously necessary.

The explanation says (D) can be fixed by removing the word "primary". That is, it's necessary that at least some steel manufacturing plants rely on electricity as a source of energy in the steel-manufacturing process. Is this really the case?

Checking with the negation technique, we get "no steel manufacturing plants rely on electricity as a source of energy in the steel-manufacturing process".

If electricity can't be used for the steel-manufacturing process, would there still be any electric bills for the steel-manufacturing plant? If not, Can the heat-converted electricity be of any other use? If so, would that use necessarily not save any money?

The premise doesn't state that the electric bills of steel-manufacturing plants result from the steel-manufacturing process. It may result from other different process that's part of the plant, and with the electric bills that come with it being reduced, money may be saved.