The first ionization energy of magnesium is smaller, as compared to that of elements X and Y, but higher than that of Z. The elements X, Y and Z, respectively, are :

The first ionization energy of magnesium (Mg) is 737.7 kJ/mol. We need elements X and Y with higher ionization energies and element Z with a lower ionization energy than Mg.

The relevant ionization energies are approximately: Ar (1520.6 kJ/mol), Cl (1251.2 kJ/mol), Ne (2080.7 kJ/mol), Li (520.2 kJ/mol), and Na (495.8 kJ/mol). Both Ar and Cl have higher IE than Mg, while Na has a lower IE.

Option (3) gives X = Ar, Y = Cl, Z = Na. Both Ar and Cl have ionization energies greater than Mg, and Na has a lower ionization energy. This satisfies all the given conditions.

The correct answer is option (3): argon, chlorine and sodium.