When performing an inspection, what is a major concern if there is no N number on the aircraft?

The absence of a registration number, commonly known as the N number, on an aircraft is a significant concern because it directly relates to the aircraft's legal status and compliance with regulatory standards. Without the N number, the aircraft may not be considered airworthy.

The N number serves as the unique identifier for the aircraft in the Federal Aviation Administration (FAA) registry, and it also establishes the aircraft's compliance with safety regulations, ownership, and eligibility for operational use. Without this identifier, there is uncertainty about the aircraft's licensing, which raises immediate questions regarding its airworthiness—its ability to be safely flown according to FAA regulations.

While other options address various aspects related to aircraft ownership and insurance, they do not capture the critical importance of airworthiness and regulatory compliance as directly as the absence of the N number does. An aircraft must have appropriate marks and be registered properly to be considered airworthy, making this the best choice in the context of the question.