Answer:
a. In conclusion, due to the vast catalog and the fact that Stephen King is still writing, it is impossible to declare that any of Stephen King’s works as his best.
Explanation:
The conclusion paragraph is the restatement of the main argument(s) made in the essay. This means that whatever has been discussed in detail in the essay's body paragraphs, the conclusion merely sums up and restates the main point(s).
And on that note, the best start of a conclusion paragraph about Stephen King's best work will be to state that there cannot be any best work by the author as he is still actively writing novels, thereby making it possible to produce better works in the coming future too.
Thus, the correct answer is option a.