The correct answer is C. Analyze as it best replaces "carefully look over" in the sentence. Although the other answers are reasonable, it is the minor differences in denotations that make "Analyze" correct. It is not "Formulate" as it means to create something, rather it be an idea or physical object. "Differentiate" tends to mean more of a compare/contrast then the context clues imply here. Finally, "Calculate" generally means to solve something.
