Lanthanide contraction is the steady decrease in size of atoms and ions of rare earth elements with increasing atomic number from lanthanum. (Atomic number =57 ) It is caused by poor shielding effect of 4f -electrons. In row 3 , we have 4f -electrons which do not shield good, causing a greater nuclear charge. This greater nuclear charge has a greater pull on electrons. Thus, we can say that decrease in atomic radii is the effect of lanthanoid contraction in lanthanoid series. Hence, option (a) is correct.