The really important cause of the recent increase in renewable energy sources is new technology.

revise the sentence to remove vague, empty words.
a. delete “renewable.”
b. delete “recent.”
c. delete “new.”
d. delete “really.”

Since we have to choose from the multiple choices, we have to sort it out option by option. The answer is not A because the sentence itself concerns renewable energy. If we delete recent, we will lose the correct meaning of the sentence concerning the increase related to time. Therefore, B is incorrect. C is not correct, as well. The sentence concerns new technology, not just any technology. Then, the correct answer is D. Indeed, "really important" is not a correct usage.  

i think that the answer is substandard.

“way of writing” should be replaced with “writing style"

“idea” should be replaced with “purpose.”

maybe   “people from a long time ago” should be replaced with “elizabethans.”