The correct answer is D. The excerpt should be revised because it contains a dependent clause that leaves the writer's thought unfinished.
Because the endangered panthers in the forests need large, connected territories to roam. This is a sentence fragment; sentences that begin with 'because' are typically sentence fragments, with a few exceptions (as you can see in the next sentence I wrote).
Because the sentence is grammatically incorrect, it needs to be revised. Therefore, the answer is either C or D. The only question is whether or not the sentence contains an independent clause or a dependent clause. An independent clause is a part of a sentence that can be a sentence on its own. For instance, in the sentence 'We should conserve nature, because the endangered panthers in the forests need large, connected territories to roam.', the independent clause is 'We should conserve nature' because it can stand on its own.
Because this excerpt is a fragment and therefore cannot stand on its own as a complete, grammatically correct sentence, it cannot be an independent clause. The correct answer is D, because the excerpt is a dependent clause and needs to be revised.
Further testing proved conclusively that the cowpox virus was able to build immunity against smallpox. Using his theory, similar vaccines were later created for diseases such as yellow fever, mumps, rubella and tetanus.
