Rank them in order of increasing ionization energy: Sr, Si, S, F.
Across a period from left to right, the ionisation energy increases. This is due to the increase in nuclear charge having a greater pull on the electrons and therefore more energy is required to remove electrons. Going down a group, the ionisation energy decreases.
Ionization energy generally increases moving from left to right across an element period (row). This is because the atomic radius generally decreases moving across a period, so there is a greater effective attraction between the negatively charged electrons and positively-charged nucleus.
Learn more about ionization energy here:
