Answer: D. barium
Step-by-step explanation:
Atomic size of an atom is defined as the total distance from the nucleus to the outermost shell of the atom.
On moving from left to right in a period, more and more electrons get added up in the same shell and the attraction between the last electron and nucleus increases, which results in the shrinkage of size of an atom. Thus, decreasing the atomic radii of the atom on moving towards right of the periodic table.
On moving from top to bottom, there is an addition of shell around the nucleus and the outermost shell gets far away from the nucleus and hence, the distance between the nucleus and outermost shell increases. Thus, increasing the atomic size of the atom.
Thus Barium has the graetest radius.