Answer:
Ca
Explanation:
Calcium atom has two electrons that are loosely attached to its outermost energy level. It easily looses these electrons during reactions than the other three elements ( aluminium,sodium and oxygen).
Ca₍aq₎⇒ Ca²⁺₍aq₎+2e⁻
Calcium ionizes by losing 2 electrons from its outer shell to form an ion with a charge of 2+
