The answer is: Mg.
Sulfate anion (SO₄²⁻) has negative charge, so element X must be element with positive charge +2.
Magnesium (Mg) is metal from 2. group of Periodic table of elements, it lost two electrons to form magnesium cation (Mg²⁺).
Magnesium has atomic number 12, which means it has 12 protons and 12 electrons.
It lost two electrons to form magnesium cation (Mg²⁺) with stable electron configuration like closest noble gas neon (Ne) with 10 electrons.
Electron configuration of magnesium atom: ₁₂Mg 1s² 2s² 2p⁶ 3s².
Electron configuration of magnesium ion: ₁₂Mg²⁺ 1s² 2s² 2p⁶.
