Answer:
D. Expert Opinion.
Step-by-step explanation:
Expert opinion can be defined as a belief or thought shared by a person on a particular subject, in which the person is knowledgeable. A specialist or expert interprets something, in which he/she has experience regarding therapy, service, etc.
In the given instance, if one finds that the source one quoted in research is a specialist or expert having multiple degrees, and has directed some of the best schools, then the quote of such source is an expert opinion.
Therefore, option D is correct.