This is what I found from other posts with this question:
Cannot have ‘that’ after comma. Hence, A is wrong. Further, in this case, the "which" is trying to refer to "several posts", which is way too far away from the comma to be acceptable. Hence D is wrong.
B is wrong because 'includes' is singular and posts to which it is referred is plural. E is awkward.
Hence answer is C.
Thanks for your help
how did you find the other posts?
Im not familiar with the search process. For example, how can I know if my question was already posted by someone else?