Answer
Observation.
Work Step by Step
The statement "Neon is an inert (or nonreactive) gas" is an OBSERVATION. There is no law or theory behind this fact, and it can be easily observed through experimentation.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.