First ionisation enthalpy of an element is 756 kJ/mole. The change in enthalpy for the process  Mg → M2+(g) will be

GivenI.P1 of ‘M’ = + 756 kJ/mole ;I.P2 of ‘M’ must be > 756 kJ/mole ;Thus sum of I.P1 and I.P2 will be > 1512 kJThusMg →M2+g, ∆H >1512 kJ ;By inspection, the only option which is greater than 1512 kJ is Option (2) ;