This market resolves once any nuclear weapon is detonated. (Including tests and civilian uses.)
The blast must be independently confirmed by multiple sources and not subject to significant uncertainty as to whether it took place.
For reference, the most recent nuclear test was in 2017 by North Korea. North Korea is the only country confirmed to have detonated a nuclear device since 1998, though there are suspicions of more recent secret tests by China and Russia.
Ok hold on, so this was obviously a nuclear explosion, right?
@StevenK I've added a "not a country" answer that would cover all 3 of those things. (Unless SpaceX was doing it in close cooperation with the US government, or ISIS was being secretly backed by some other country's government, etc.)