The correct option is iii) Higher ionization potential. The potential energy of Sodium is higher than Potassium because of the smaller atomic size.
Sodium has three orbits, and potassium has four orbits. Lesser the distance of orbits from the nucleus, the higher the energy is required to remove the outermost electron, and vice versa. Therefore, more energy is required to remove an electron from sodium than from potassium. Hence, the ionization energy of sodium is higher. The electron in Potassium is also more affected by shielding due to more shells, further weakening this attraction. This means that less energy is needed to remove the outermost electron, and therefore, the ionisation energy is lower.