In which circumstance may a second in command not sign the preflight checklist under 14 CFR 121?

The correct answer is that a second in command may not sign the preflight checklist because they are not authorized to do so. According to 14 CFR 121, it is designated that only the pilot in command is responsible for the finalization of the preflight checklist. This regulation emphasizes the importance of accountability and responsibility in flight operations, indicating that the pilot in command must ensure that all preflight duties and checks are completed.

The other scenarios presented do not align with regulatory guidelines. For instance, while having a special inspection or the pilot in command being unavailable might raise operational concerns, they do not directly pertain to the authorization required for signing of the preflight checklist. Similarly, lacking a commercial pilot certificate would limit the second in command's role, but the key factor remains the lack of authorization to sign the preflight checklist itself. Thus, the preflight checklist sign-off is strictly regulated to maintain safety and order within the cockpit.