Mg
The period 3 elements are those in the 3rd row of the periodic chart and they are sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, and argon.
Looking at a chart of the first ionization energies for all of the elements, it is fairly obvious that of the 8 candidates, the only ones that are possible are magnesium, aluminium, and silicon. And looking at those 3 elements in detail reveals that magnesium has 737.7, 1450.7, and 7732.7 kJ/mol as its first three ionization energies, which is a perfect match.
