What is the correct phraseology for instructing an enroute aircraft at 9,000 feet to climb to 1,000 feet?

The correct phraseology for instructing an enroute aircraft to climb to a specific altitude is "Climb and maintain one thousand." This statement provides clear and unambiguous guidance for the pilot regarding their altitude change.

Using the phrase "climb and maintain" indicates that the aircraft should not only ascend to the specified altitude but also maintain that altitude once it has been reached. This is vital for ensuring safe separation from other aircraft and for following air traffic control (ATC) directives.

Furthermore, phraseology in aviation is standardized to prevent misunderstandings due to language differences or miscommunication. The phrase "one thousand" is typically used in aviation contexts where numbers are pronounced clearly to avoid any confusion with similar-sounding numbers.

Other options may lack the precise, standardized communication required in air traffic situations. Options like "ascend to 1,000," "climb to one thousand," or "go to one thousand" do not convey the maintenance aspect of the altitude or may not align with the expected terminology used in air traffic control communications. It’s crucial that all instructions to pilots are unambiguous and follow established phrasing to ensure safety and efficiency in aviation operations.