The answer is either B or C. I would be able to determine which is correct if I were given the passage.
The importance of using a variety of sentence structures is to change up your writing. To make the reader engaged in what you have to say. If your sentence structure is weak and does not vary the reader might get bored. It also better shows your topic.
hope this helps! have a good day!
The adverb is either because it helps describe the sentence more.
Green is the first one
Light blue is the second
Purple is the third