This question focuses on academic writing skills. The correct answer is 'C. The clarity and coherence of the argument' because effective academic writing should present ideas clearly and in an organized manner. 'A. The use of complex sentences' can sometimes impede clarity. 'B. The inclusion of personal opinions' is often discouraged in academic writing unless it is a reflective piece. 'D. The use of decorative language' is not a key factor and can detract from the严肃ness of the academic work.