In this sentence, the clause starting the sentence is a modifier for the noun phrase "the woolly adelgid" and the second modifier "which causes needles to change color from deep green to grayish-green" in the sentence describes the result of the clause "the woolly adelgid retards tree growth" and so, is supposed to modify the entire clause.

Option A - Incorrect
This option is wrong because it places the wrong clause after the modifier "by sucking sap from young twigs of hemlock tree..." This causes confusion in the meaning which is totally wrong.

Option B - Incorrect
It has the same issue as option A. The clause "tree growth is retarded by the woolly adelgid..." makes it seem as if tree growth is what is sucking the sap.

Option C - Incorrect
The verb form "dropping" used at the end part of this sentence is not parallel to the previous verb "to change." This is highly incorrect. Also, the use of the pronoun "which" in the sentence is ambiguous. It refers to "tree growth" instead of "the woolly adelgid."

Option D - correct
This option has an acceptable structure. It puts the phrase "the woolly adelgid" right after its modifier. It also uses the correct form of verb to explain the action thereby conveying the meaning accurately.

Option E - Incorrect
The pronoun "this" used in the sentence doesn't have any antecedent creating an ambiguity in the meaning of the sentence.

okay i can see