Keeping in view the size of atoms, which order is the correct one:

Explanation

Ba and magnsium are in same group . Ba comes after Mg it means it has more shell so it has large atomic sizeAtomic Radius:Atoms are so small that it is impossible to see an atom even with a powerful optical microscope. The size of a single atom therefore cannot be directly measured. However, techniques have been developed which can measure the distance between the centres of two bonded atoms of any element. Half of this distance is considered to be the radius of the atom. In the periodic table, the atomic radius increases from top to bottom within a group due to increase in atomic number . This is because of the addition of an extra shell of electrons in each period. In a period, however, as the atomic number increases from left to right, the atomic radius decreases . This gradual decrease in the radius is due to increase in the positive charge in the nucleus . As the positive nuclear charge increases, the negatively charged electrons in the shells are pulled closer to the nucleus. Thus, the size of the outermost shell becomes gradually smaller. This effect is quite remarkable in the elements of longer periods in which "d" and "f " subshells are involved. For example, the gradual reduction in the size of Lanthanides is significant and called Lanthanide Contraction.