The correct answer is 16. Concept:Atomic number:The atomic number of an atom is equal to the number of protons in the nucleus of an atom or the number of electrons in an electrically neutral atom.Atomic number = Number of protonsFor example, in a magnesium atom, there are 12 electrons and 12 protons.Explanation:When the element X gains 2 electrons to form an anion X2-, it is no more in an electrically neutral state.An atom is said to be electrically neutral when it has the same number of protons and electrons. Element X has the atomic number 16. So, it has 16 protons and 16 electrons.The anion X2- has 16 protons and 18 electrons. Since the number of protons is the same, the atomic number remains the same i.e., 16.