Which has highest second ionisation energy?
Electronic config of Na(+) is noble gas config. . so its very difficult to remove an electron so it will have very high 2nd ionisation enthalpy in case of Mg(+) and Al(+) its not noble gas config so 2nd ionisation enthalphy will be realtively less
Now we have Na(+) and K(+)
As we know that the size of K(+) is more than that of Na(+) so ionisation enthalpy will be more for Na(+)
So 2nd ionisation enthalphy will be the highest for Na .