What phraseology should be used to decline an operational request?

Using the phrase "Unable (requested operation)" is the appropriate choice for declining an operational request because it clearly communicates to the party making the request that their request cannot be fulfilled, while maintaining professionalism and clarity. This phraseology is also consistent with established aviation communication standards, which prioritize clear and concise language.

The reason this option stands out is that it utilizes standard aviation language that is easily understood by all parties involved. It conveys the necessary information without ambiguity, allowing the other party to comprehend the situation quickly. This is crucial in an operational context where miscommunication can lead to safety issues or operational inefficiencies.

The other options, while they might convey a similar message, do not adhere as closely to the preferred terminology used in air traffic communications. They may come across as overly blunt or lacking the required professionalism, which can affect the dynamics of communication in a sector that relies heavily on collaborative efforts among various stakeholders.