The decreasing order of the second ionization energies of K, Ca and Ba is

Explanation

The second ionization energy is the energy required to remove a second electron from an atom. Potassium (K) has the highest second ionization energy because once the first electron is removed, it attains a noble gas configuration, making it very stable and requiring more energy to remove another electron. Calcium (Ca) follows because although it also attains a relatively stable state, the electron is removed from a higher energy level. Barium (Ba), being in a lower position in the periodic table, has a larger atomic size and increased shielding effect, reducing the energy required to remove a second electron. Hence, the order is K > Ca > Ba.