The element having the greatest difference between its first and...

According to given options, except for potassium and Sc rest, others are alkaline earth metals with an outer configuration ${ns}^{2}$.
In the case of potassium, the outer configuration is $1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}3{p}^{6}4{s}^{1}$.
First ionization enthalpy is low, but once an electron is removed, it will attain noble gas configuration and will become more stable. Hence, removing the second electron is difficult and requires a large amount of energy.