Due professional care in auditing requires that internal auditors base their opinions on sufficient factual evidence. This ensures that their conclusions are reliable, credible, and defendable. The effectiveness of internal controls (option A) is a part of what auditors examine, but it's not the sole basis of professional care. Expressing opinions only when a consensus with top management is achieved (option B) undermines the independence and objectivity required in auditing. While basing opinions on experience and free of all bias (option C) is important for maintaining objectivity, it does not explicitly capture the necessity of grounding opinions in factual evidence, which is fundamental to due professional care.