Answer:
a. Conducting research activities in a private setting
Explanation:
This method does not necessarily guarantee the privacy, but it is the only one that is considered as a method to protect it from the rest of remaining options.
One of the way a breach of privacy could occurs during research is if there is other individuals present during the information gathering place (especially if that individuals know/recognize the subjects and can spread it within the subject's social circle.)
Conducting the research in private setting could prevent such occurrence from happening since we can control who can and cannot be present within the research space.