I am thankful that I am not in a position to make that determination but I think it would depend on the yield of the nuclear device used as well as the target. If the Russians "just" used a tactical nuclear device with a low yield and minimal fallout on a purely military target then I think the West might respond by (1) increasing lethal aid to the Ukrainians, (2) strengthening sanctions and (3) attacking a series of DNR/LPR targets in occupied Ukraine as well as one or more medium-to-high value Russian military assets in Crimea or the Black Sea.
Cyber attacks might also be on the table. In February, Biden was presented with a series of options that would have resulted in the degradation of Russia's power grid as well as chaos in their transportation network by stopping trains in their track. I believe such options were deliberately leaked at the time to show that Biden was taking a more "measured" approach and that the US had the capability to carry out these kind of attacks should Russia escalate.
https://www.nbcnews.com/politics/national-security/biden-presented-options-massive-cyberattacks-russia-rcna17558
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