I'm not sure but i think you put the answer in your question.
Both A and C
She is wealthy, the last answer
Answer: I would contend that the right answer is the E) "... since they provide fascinating evidence about the creative processes in which this great Modernist engaged."
Explanation: Just to elaborate a little bit on the answer, it can be added that the other options feature sentences that end with a preposition ("engaged in"), and although this is permissible in the English language, it is not the most formal option. In the example, a good way to correct it is to replace it by "in which." I would suggest changing the sentence so it reads "Although the author himself declared them unpublishable, the first appearance in print of 40 poems by T.S. Eliot is a significant event, since they provide fascinating evidence about the creative processes in which this great Modernist engaged."
