What is a requirement for an aerial work service or air taxi service carrying an external load?

For an aerial work service or air taxi operation involving the carrying of an external load, it is essential that the operation is conducted in accordance with the aircraft's approved configuration. This is where the relevance of a supplemental type certificate (STC) or airworthiness approval comes into play. An STC or airworthiness approval indicates that the aircraft has been modified in a manner that meets regulatory safety standards, ensuring it can safely carry external loads.

When carrying external loads, special considerations such as the aircraft's structural integrity, performance limitations, and loading procedures must be adhered to. The STC provides specific information and guidelines that govern these modifications, confirming that they are safe and permissible under regulatory standards. This is crucial not just for the safety of the operation but also for compliance with aviation regulations.

Other choices do relate to operational safety and compliance but do not encompass the regulatory framework concerning external load operations as comprehensively as the requirement for STC or airworthiness approval. For example, documentation in the flight plan or notifying ATC might be necessary for operational procedures, but these actions do not address the core requirement concerning aircraft modifications and safety as outlined in the right answer.