Which subset does the number 0 belong to?

Answer 1

To any set that contains it!

It belongs to {0},

or {45, 0, sqrt(2), pi, -3/7},

or {0, bananas, France, cold}

or all whole numbers between -43 and 53,

or multiples of 5,

or integers,

or rational numbers,

or real numbers,

or complex numbers,

etc.