Answer: K
EXPLANATION: first ionization energy means to remove the very first electron from the outermost shell of an atom.
Now, why do we need energy to remove electron from atom? because nucleus is holding all the electrons.
So, we give energy to pull electrons away from the hold of nucleus. The stronger the hold of nucleus, the difficult is removal of electron.
Out of K and Rb, the outermost electron in K is in 4th shell while in Rb is in 5th.
4th shell lies closer to the nucleus. so there is high hold on 4th shell electrons than 5th shell electrons. so, removal of electron from 5th shell is easier than from 4th shell. So, less energy required in Rb but higher for K.
