Ionization energy is said to be the amount of energy required to remove a valence electron from the atom of an element to become a gaseous specie. 

Ionization energy decrease as you move down the group and increase across the period in the periodic table. Consequently, the elements under consideration are in the same period, period 3, ranging from Aluminium to Silicon to Phosphorus to Sulphur. i.e Al < Si < P < S.