That's an odd question. I think it is more about eliminating wrong answers than actually picking the correct answer.
A) This uses "was" as a helping verb. The pronoun refers to "energy" so if you substitute the antecedent for "it" the setence reads: "...than energy was in 1990." This is incorrect. We're left dangling. Energy was what? We don't know what energy was because we're missing the verb in this part of the sentence. It could be fixed like this "...than was generated in 1990." but that isn't an answer choice.
B) We have to determine the antecedent for the pronoun "it". "It" appears to refer back to energy, but there isn't a verb in the second half, so it's not correct. "Was" is a helping verb that usually is found with a verb ending in -ing. In B), the sentence doesn't work because there isn't a verb for "was" to help.
C) This is correct because "was" is not being used as a helping verb in this sentence. Was is THE actual verb. "the case" is the direct object of the verb "was". Still, this isn't a great sentence, but is the "best choice" compared to the others.
NOTE: When we see words that are generally helping verbs, we have to determine if the word is being used as a verb or helping verb. This determination can make the sentence right or wrong.
D) The only difference between B & D is that in D, the authors move "now" to the front of the answer. This still does not fix the problem found in B.
E). This is similar to C, but they use "then" rather than "than" which is incorrect.
There are hopeful signs that we are shifting away from our heavy reliance on fossil fuels: more than ten times as much energy is generated through wind power now than it was in 1990.
A) generated through wind power now than it was
B) generated through wind power now as it was
C) generated through wind power now as was the case
D) now generated through wind power as it was
E) now generated through wind power then was the case
Answer is C, please explain!
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.