Which aspect of a shared-use airport's operation is subject to Part 139?

In the context of shared-use airports, Part 139 regulations are specifically applied to areas that are involved in passenger-carrying operations. These regulations are designed to ensure safety at commercial service airports that accommodate carrying passengers. This includes requirements for the airport's infrastructure, maintenance of runways, taxiways, and safety management systems that are relevant to operations involving commercial air transport.

While cargo operations, military operations, and other areas may have their own regulations, the stringent safety and operational requirements dictated by Part 139 focus primarily on segments of the airport that service passenger flights. This is critical for maintaining the safety standards necessary for airlines and their passengers. Hence, the correct choice emphasizes the specific portions of the airport that are engaged in such operations, highlighting their compliance with regulatory safety mandates.