Answer:
C. Written Reports
Step-by-step explanation:
In order for a research to be considered as 'Formal' , it need to provide a clear systematic approach along with reliable evidence leading up to the conclusion.
Observations, interviews, and personal ideas can be included in formal research, but that's not necessarily be the main criteria.
Written reports on the other hand, is always a must in order to provide the readers with that comprehensive details on how the experiment is conducted along with set of explanations that make the researchers arrive to a certain conclusion.