Which of the following is easily formed?
 

The elements which lose or gain electrons very easily readily form an ionic bond. 
Potassium bromide is easily formed.
Among the ions ${\mathrm{K}}^{+},{\mathrm{Mg}}^{2+}{\mathrm{Ca}}^{2+}$, the ionization energy is minimum for ${\mathrm{K}}^{+}$ ion. Also, among fluorine, chlorine, and bromine, the magnitude of the electron gain enthalpy for bromine is minimum.