Explanation:
Ionization energy is the energy necessary for a gaseous atom or ion to remove an electron.
Larger is the size of an atom easily it is able to remove an electron as valence shell is away from the nucleus. So, there occurs no force of attraction between the nucleus and valence electrons.
On the other hand, smaller is the size of atom then more is the force of attraction between its nucleus and valence electrons. So, more energy is required to remove an electron. This causes high ionization energy.
Therefore, down the group size of atoms increases this results in decrease in ionization energy.
Since, Li, Na, K and Rb are all group 1 elements. Hence, their order of first ionization energies from highest to lowest will be as follows.
Li > Na > K > Rb
