Which air service does not belong to the RAF's historical formation?

The Royal Air Cadets is indeed the entity that does not belong to the historical formation of the RAF. The RAF, or Royal Air Force, was officially formed in 1918, and its predecessors included the Royal Flying Corps and the Royal Naval Air Service. Both of these were instrumental in the development of military aviation in the UK prior to the establishment of the RAF, representing the army and the navy's efforts in air operations.

The Naval Air Service also served before and contributed to the foundation of the RAF, focusing on maritime aviation. The Royal Flying Corps was specifically the aviation arm of the British Army and played a significant role in World War I.

In contrast, the Royal Air Cadets, established during World War II as part of the Air Training Corps, is a youth organization designed to provide young people with opportunities in aviation and leadership training. While it is associated with the RAF today, it does not have the same historical roots in the formation of military air services that the other options do. This distinction makes it the correct choice for the question regarding historical formation.