Which phraseology is used to determine the altitude of an aircraft?

Answer: C — Say altitude

The phrase "Say altitude" is commonly used in aviation communication to prompt an aircraft to provide their current altitude. This phraseology is clear and concise, ensuring that pilots understand the request to communicate their altitude without ambiguity. It aligns with standard aviation practices which prioritize clarity in communication, especially in critical operations such as altitude tracking. The use of this specific phrase helps maintain situational awareness among air traffic control and nearby aircraft by ensuring accurate altitude information is exchanged promptly. Other options may vary slightly in intent but lack the established protocol or clarity found in "Say altitude," which can lead to misunderstandings during communications.