why is c correct?

The conclusion of the argument is the claim being referred to in the question stem: "Hence, doctoral dissertations should not be required in the humanities." You can see that "hence" introduces the conclusion. The entire passage is supporting this statement. We are told that doctoral dissertations should not be required in the humanities because Ph.D. programs are valuable only if they inculcate good scholarship and expedite the student's full participation, but a doctoral dissertation in the humanities does not allow for either of these. This is why answer choice (C) is correct: "It is what the argument is attempting to establish," i.e. it is the conclusion of the argument. Remember, what is a conclusion? It is the point that the argument is set out to make or establish. So, answer choice (C) is the correct answer. Hope that clears things up! Please let us know if you have any other questions.

out of curiosity can "hence" introduce a premise such that "clearly" can introduce both?