If the gas in this flask is non reactive and monatomic which elements would it be

All inert gases are considered non-reactive and monoatomic gases.
This is because, their outer shell is saturated with electrons, so they neither need to lose, gain or share electrons with any other atom.

The list of inert gases is:
Helium, neon, argon, krypton, xenon and radon.

Since you haven't provided sufficient information, I cannot tell which of these gases is your answer. The only thing I can assure is that it is one of the mentioned inert gases.