By the year 2008, Harry Potter, J.K. Rowlings' gripping book

1. If the answer that your source gives as correct really is option D, then you are looking at another example of faulty preparation material. (It's hard to know in this question whether the underlined or the non-underlined part is worse: I assume that you have transcribed it accurately.)

2. You would have to reject option D for reasons similar to those for which you would reject option A. Let's look at it:

3. The problem in the given sentence and option A (apart from the absence of a comma after VOLDEMORT) appears in the final phrase making it more than that of almost any other English book ever written.
First, it is unclear what MAKING is modifying.
Second, if we accept that IT refers to Harry Potter (and that's a long shot), that book is then MORE what? More successful? More profitable? There is no way of knowing.

4. Furthermore, there are problems in the non-underlined part of the sentence.
First, (though perhaps this is simply a bugbear of my own), what exactly is an ENGLISH BOOK? A book offering to give instruction in English? A book with English background? (There is no such thing.) A book written in English? Since the sentence wants, I imagine, to say that this novel is a book written in English, that is what it should have said.
Second, if Harry Potter is MORE something than any other English book, it is MORE something than any other English book ever written. EVER WRITTEN is therefore redundant.
Third, the pronoun THAT has no antecedent.
Fourth, the first 12 words of each option are identical, and so should not appear at all. Nor should the final MORE THAN, which appears in every option.

5. Since option D has the same problem with MAKING, IT, and MORE that option A has, you could eliminate it.

6. In option B, apart from other problems already mentioned, the antecedent of WHICH is unclear.

6. In option E, the problem with MORE reappears.

7. The only remaining option is C, so it is perhaps the best answer, though a very bad best. MORE does refer to the 200 million copies in print, but the problems in the non-underlined part of the sentence remain, especially MORE THAN THAT OF. What precisely is THAT? It is impossible to know. So I'd tend to say that this question actually has five incorrect options.

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can you cite the source of this SC?