the 1st ionization enthalpy of calcium is greater than that of potassium because for calcium

a- the electron is removed from s-subshell

b- the effective nuclear charge is higher

c- the electron is removed from a p-subshell

d- the removal of electron does not give noble gas configuration

The correct option is b.
As we move from left to right in periodic table, the effective nuclear charge on valence electron increases because of addition of extra proton in nucleus. Hence element in the right like Ca require higher amount of energy to remove valence electron as compared to K .