Which is the best definition of closure?

Number sets are closed under an operation when you perform the operation and the result is a number that is not a real number.

Number sets are closed under an operation when you perform the operation and the result is a member of the opposite number set.

Number sets are closed under an operation when you perform the operation and the result is a member of a related number set.

Number sets are closed under an operation when you perform the operation and the result is another member of the same number set