The domination factor responsible for the decreasing Ionization energies of the elements of  moving down the group is :

The spacing between the nucleus and the electron in the valence shell increases as an atom's size rises. As a result, there is less attraction between the nucleus and the electron of the valence shell. Therefore, less energy is needed to remove the valence shell electron. So, it is called the atomic radius. 

Hence, the correct option is A.