Final answer:
Nurses in Ontario are required to self-report frequent medication errors to the College of Nurses of Ontario as it poses a risk to patient safety and indicates a potential lapse in maintaining the standards of practice.
Step-by-step explanation:
The question pertains to the professional responsibilities of nurses in Ontario, specifically those related to self-reporting to the College of Nurses of Ontario (CNO). In such a context, the circumstance that is mandatory for nurses to self-report is d. frequent medication errors. Medication errors can significantly impact patient safety, and the CNO needs to ensure that all nurses meet the standards required for safe practice. Whereas pardoned criminal offences may not need to be reported depending on the nature and relevance to nursing practice, and communicable diseases are reported to public health rather than the CNO, nurses do need to report if they have been involved in work related disciplines that are serious in nature or if there is a pattern of unsafe practice. However, frequent medication errors specifically indicate a potential risk to patient care and thus must be reported as part of regulatory requirements to maintain a safe practice environment.