It's obviously {}, duh! ;) (In one of the ways of constructing the natural numbers within ZFC set theory in mathematics, one identifies 0 with the empty set {}

It's obviously {}, duh! ;) (In one of the ways of constructing the natural numbers within ZFC set theory in mathematics, one identifies 0 with the empty set {}, 1 with {{}}={0}, 2 with { {}, {{}}} = {0,1}, and so on.)