Which of the following represents an ion?

Ca

Na^+

Mg

^4He

The correct option Na⁺.

Na⁺ is an ion because it has a charge. An atom or molecule with a charge of either positive or negative is an ion. there are two types of ions: Cations that are positively charged ions and they get positive charge as a result of losing electrons, and the second type is anion that are negatively charged ions and they negative charge as a result of gaining electrons.