Which of the following indicates that an operational request has been denied?

The correct statement indicating that an operational request has been denied is the phrase "Unable (requested operation)." This phrase is concise and directly conveys that the air traffic controller is unable to accommodate the requested operation. It is a standard response used in air traffic communications to indicate that a pilot's request cannot be fulfilled due to various operational constraints, thereby clearly informing the pilot of the denial without ambiguity.

While other phrases might also suggest a denial, "Unable (requested operation)" is particularly straightforward and consistent with the established phraseology used in air traffic control communications. It's important to note that effective communication in aviation is critical to ensure safety and operational efficiency, and this phrase effectively fulfills that requirement.