Answer:
C. The investigator uses the informed consent process to explain her institution's method for guaranteeing absolute confidentiality of research data.
Step-by-step explanation:
If you want to apply the best possible practice of convincing participants of a survey that their data is completely safe, you should leave no room for doubt. The answer is obviously not A, B or D because the investigator is deliberately telling the participants that he's not sure if their data will be completely confidential.