Answer: D. "You offer great evidence in your essay but I'm not sure what your argument is; make sure that is clear to the reader."
Explanation: All other options draw attention to specific, technical issues in the essay that need to be addressed: sentence length, run-on sentences and errors in verb use. Only option D refers in a general way to the whole text and presents a global problem, of which the details will have to be discovered and sorted out by the student.
