A study promises confidentiality: the researchers know who said what but will not disclose it. Another promises anonymity: nobody, including the researchers, can link a response to a person. Why does the difference matter for what a participant is actually risking?

C. Because confidentiality keeps a link that anonymity never creates.

Confidentiality is an undertaking about behaviour — the key exists and can be subpoenaed, stolen or carried out by a departing lab member — where anonymity removes the possibility rather than promising against it. Longitudinal work must keep a link, and the honest position there is to promise confidentiality and secure it, not to call it anonymity.