An obscure novel from the thirteenth century has been attributed to

An obscure novel from the thirteenth century has been attributed to Author X, although whether she was the sole author of the long work is often disputed. The year of completion has also been disputed but should be accepted as 1221, because in this year Author X wrote a diary entry about her joy at having completed the work. In this diary entry she also mentions a specific character who appears only in the last chapter of the book. While doubts may remain about certain aspects of the book's authorship, there is no doubt about its chronology.

Which of the following, if true, most strongly supports the argument above?

A. In several diary entries in the year 1220, Author X mentioned that she was lonely working on the obscure novel but that it was almost complete.
B. For record-keeping purposes, Author X had a court scribe copy her diary, so a second copy of her diary verifies the authenticity of her entry about the obscure novel in 1221.
C. No writers other than Author X wrote interior portions now accepted as part of the obscure novel after Author X had finished her portion in 1221.
D. Early in 1222, a political coup took place that would have made life so turbulent for Author X that it would have been virtually impossible for her to continue to edit and add to the obscure novel.
E. The last chapter of the book was the last one Author X wrote.

Answer is C
Here is the reason
In the argument it is given that "In this diary entry she also mentions a specific character who appears only in the last chapter of the book" and this event took place in 1221 .So if there is no other writer wrote portion after she finished writing the last chapter or so .It make the argument more valid the author X must have completed the book in that year itself.