Ionisation energy is the energy required to remove an electron from an isolated gaseous atom. Ionisation potential decreases as we move down in the group. The atomic number increases in a group and hence the number of shells also increases. Thus, outermost electrons are far away from nucleus and can be removed easily. Hence, statement A is correct The outer electron in potassium is in 4s-orbital, which is further away from the nucleus than the 3s-orbital of sodium. This means that less energy is needed to remove the outermost electron. Therefore, first ionisation potential of sodium is greater than potassium. Hence, statement (b) is also correct.