Official Solution:
Found only on Stephens Island in New Zealand, A.C. O'Connor discovered the ngaio weevil, which is classified as nationally critical.
A. A.C. O'Connor discovered the ngaio weevil, which is classified as nationally critical
B. the ngaio weevil with a classification of nationally critical was discovered by A.C. O'Connor
C. the ngaio weevil, whose status is classified as nationally critical, was discovered by A.C. O'Connor
D. the ngaio weevil, which is classified as nationally critical, was discovered by A.C. O'Connor
E. A.C. O'Connor discovered the ngaio weevil with a classification of nationally critical
Let's review our answer options. Our answers are split around two issues. We must determine what noun should follow the adjectival phrase that begins the sentence: A.C. O'Connor or the ngaio weevil. We must also determine if we should use a relative clause or a prepositional clause to describe the weevil. Additionally, if we use a relative clause, we must determine which relative pronoun (which or whose) should be used.
We must look at the portion of the sentence that is not underlined to determine which noun we need to use. The adjectival phrase is "found only on Stephens Island in New Zealand." This phrase is describing the noun that immediately follows it. Let's test it. A.C. O'Connor is found only on Stephens Island in New Zealand. That seems highly unlikely. The ngaio weevil is found only on Stephens Island in New Zealand. That makes much more sense. The noun "the ngaio weevil" should immediately follow the adjectival phrase. Options A and E cannot be correct.
The elimination of these answers leaves us with sentences that use the passive construction "was discovered." A passive construction is not always ideal, though it does have its uses. However, it is not incorrect, and using A.C. O'Connor as the subject of the sentence would make the sentence grammatically incorrect. In this situation, the passive construction is the best option, because we have no better, correct options.
Now, we must consider if we should use a relative clause or a prepositional phrase. The prepositional phrase "with a classification of nationally critical" acts as an adjective answering the question "which one?" Which ngaio weevil? The one with a classification of nationally critical. Since these weevils are only found in one place in the world, it seems unlikely that we need to specify which ngaio weevil. The prepositional phrase is not technically wrong, but it makes the sentence awkward and changes the sentence by implying there is more than one type of ngaio weevil. The prepositional phrase doesn't simply provide additional information like a relative clause does. The prepositional phrase is not the best option in this sentence. Options B and E cannot be the best answers.
Now that we know we need to use a relative clause, we must determine which relative pronoun to use: which or whose. Whose is a possessive relative pronoun. Since it is the only possessive relative pronoun in English, it can be used with animals and things. However, using the possessive relative pronoun means we must add a word describing what the weevil possesses. In this case, a status. Adding a word ignores the principle of concision. It is easier, more concise, and just as accurate to use the relative pronoun "which." "Which" simply provided more information about the ngaio weevil. Option C cannot be the best answer.
We have successfully four answer choices. Option D is the best answer.
Answer: D