Final answer:
Barium is the element that loses two electrons when becoming an ion.
Step-by-step explanation:
The element that loses two electrons when becoming an ion is barium (option a). Barium has an atomic number of 56, meaning it has 56 electrons. When it loses two electrons, it forms a cation with a charge of +2. This is because it now has 54 electrons and 56 protons, resulting in a net charge of +2.