The correct order of ability to form complexes is 

 Generally on, going down the group the size of the metal ion increases. And down the group the ability to form complexes decreases. That is, from ${\mathrm{Be}}^{2+}$ to ${\mathrm{Ba}}^{2+}$ the complex making ability decreases.

${\mathrm{Be}}^{2+}>{\mathrm{Mg}}^{2+}>{\mathrm{Ca}}^{2+}>{\mathrm{Ba}}^{2+}$

Due to small radius of ${\mathrm{Be}}^{2+}$, it has more ability to form complexes.