In which scenario is it likely that a breach of confidentiality could occur?

A breach of confidentiality is most likely to occur when sensitive data is compromised, and that is precisely the situation when a laptop containing confidential information is stolen. In this case, the unauthorized access to the sensitive data on the laptop can lead to disclosure of personal information, which violates the principle of confidentiality that researchers are obligated to maintain.

In contrast, the other scenarios involve practices aimed at protecting confidentiality. Using anonymized data reduces the risk of identification of individuals, while conducting a study solely with public data means that the information used does not involve any private or sensitive data that would require protection. Additionally, not collecting identifiers is a proactive measure to ensure that the information cannot be traced back to any individual, further safeguarding participant confidentiality. Thus, these practices help in mitigating the risks of confidentiality breaches, making the situation involving the stolen laptop the most vulnerable to such breaches.