The correct answer is answer choice B, 2. Calcium is in Column 2, meaning that it has 2 electrons in its outermost energy level. These two electrons must be lost for it to form an ionic bond with another element and become stable.
Questions like these can be easily answered if you have a periodic table on hand. Elements in Column 1 need to lose 1 electron to form ionic bonds, elements in Column 2 need to lose 2, and so on. Keep in mind, however, that elements in the center of the table are more likely to form metallic or covalent bonds, and elements in Columns 16 and 17 need to gain 2 and 1 electrons, respectively.
