Eliminate options B and E due to subject-verb agreement. The subject "skull" requires "has been found."
Eliminate A due to the awkward and wordy "in addition."
Eliminate D because it is a sentence fragment -- the subject "skull" has no corresponding verb in the sentence.
The correct answer is C.
To clarify why D is wrong, since some of the earlier posts support it, let's first look at C. If we strip away all of the "extras" from this sentence (any modifying phrases and clauses) and get to its essential core, what results is:
A skull has been found.
If you try that with D, you wind up with nothing but:
A skull.
That is a fragment!
Rey Fernandez
Instructor
Manhattan GMAT