This question asks you to pick the answer choice which most accurately re-states the conclusion of the argument. So, before turning to the answer choices, it's best to identify the conclusion. Here's a breakdown of the argument:
Premise: Novel X and Novel Y contain similar themes and situations. Premise: The authors of Novel X and Novel Y have similar backgrounds and have led similar lives. Conclusion: Therefore the similarities in the books are more likely to be due to coincidence than plagiarism.
Answer (D) correctly restates this conclusion. It fully lays out the argument that the similarities in the books is less likely to be because of plagiarism and more likely due to coincidence.
Answer (E) is incorrect for a few reasons. First, it phrases the conclusion as if it is a cause + effect argument. That is not the case here. Second, it overstates the conclusion. The passage argues that the similarities are "more likely" due to coincidence than plagiarism. Here, the statement says that suspicions of plagiarism are "very likely" unwarranted. While small, there is a difference between "more likely" and "very likely"