The species having smallest ionic radius is :

Ionic radius of ${\mathrm{Al}}^{3+}$ is equal to 53.
Ionic radius of ${\mathrm{Ba}}^{2+}$ is equal to 135.
Ionic radius of ${\mathbf{K}}^{+}$ is equal to 138.
Ionic radius of ${\mathrm{Mg}}^{2+}$ is equal to 72.