What is the correct phraseology for assigning an altitude of one thousand to an enroute aircraft currently at nine thousand?

The phraseology "Climb and maintain one thousand" is the correct choice because it specifically communicates the directive to the pilot to ascend to a specified altitude, which in this case is one thousand feet. This phraseology is standardized and widely accepted within aviation communication, ensuring clarity and precision in instructions given to pilots.

By using "climb and maintain," air traffic controllers provide clear instructions that encompass both the action required (to climb) and the altitude that must be maintained once reached. This helps prevent any misinterpretation of the instruction, which is critical for safety and effective operations in controlled airspace.

The other options lack the precision or standardized format required for altitude assignments. For example, "proceed to one thousand feet" could be interpreted as simply flying towards that altitude without the explicit instruction to climb, potentially leading to misunderstanding. Similarly, "ascend to one thousand feet" does not use the standardized phraseology and could introduce ambiguity, while "climb to flight level one thousand" is incorrect in this scenario because flight level one thousand refers to 10,000 feet, not the target altitude of one thousand feet, which is below the transition level. Hence, the clear and correct choice is to say, "Climb and maintain one