c
In general, the atomic and ionic radii increases on moving down a group but the elements of second transition series \((Zr, Nb, Mo)\) have almost same radii as the elements of third transition series \((Hf, Ta, W, etc)\). This is because of lanthanoid contraction, i.e.

imperfect shielding of one \(4f \;e^{-}\) by another.