Atom Y
This is because if we look at the atoms structure, an atom needs 3 electrons in the first ring, 2 electrons in the second ring, so and so forth. Now, as for atom z, it is a completed atom or element, meaning it does not need any electrons and it does not need to give any electrons.
While, the atom y only has 1 electron on the outer field, which means that it needs 1 more electron. And, as the definition of an ion states that it is an incomplete for unstable atom, atom y would be your answer.