Which of the following element will have maximum ionisation enthalpy?

 Ionization energy increases from left to right in the period because as we move left to right in a period, the nuclear charge increases, leading to a strong bond between the electrons and the nucleus. Thus, a large amount of energy is required to remove the outermost electron from an atom. Thus, ionization energy increases from left to right in the period.

Magnesium, aluminium, sulphur, and chlorine belong to period 3. Chlorine is at the extreme right and so it will have maximum ionisation enthalpy.