If Russia invades Ukraine, how should the US respond?

[Middle-aged Europe]

A combination of 2, 3, and 6.

[Middle-aged Europe]

Nothing, Russia has the right to a sphere of influence

No country in the world, including Russia, has a right to a sphere of influence. Countries have a of course a right to enter voluntary alliances with each other and if the Russian and Ukrainian governments would agree to such an alliance they're free to do so.

[Middle-aged Europe]

Btw, if the West places sanctions on Russia it's possible if not likely that Russia will cut off the gas supply to Western Europe. In which case emergency gas supplies from America to Europe would become necessary and apparently the Biden administration is already preparing to make such emergency supplies available:

https://www.reuters.com/business/energy/exclusive-us-talks-energy-firms-over-eu-gas-supply-case-russia-ukraine-conflict-2022-01-15/