The answer is D: The student should replace informal language with formal language.
All forms of academic writing, like an essay, should strive to always use formal language and avoid informal language. The latter is a form of language commonly used in conversation where clarity and precision are not necessary since the listener can always ask for clarification. Formal language, on the other hand, is aimed at accuracy and clarity in conveying distinct and non-ambiguous thoughts, arguments, and thesis, through sentences that are as meaningful as possible and require no further explanation or clarification. Revision of an essay should always consider this.