Which axiom shows that a class of all countable ordinals is a set? As stated in the title, which axiom in ZF shows that a class of all countable (or any cardinal number) ordinals is a set?
Not sure which axiom, that's all.

A combination of them, actually. The proof I've seen requires power set and replacement, and I think union or pairing, too, but I can't recall off the top of my head. I can post an outline of that proof, if you like.
