Due to lanthanide contraction

Explanation :-  
Lanthanides are elements from Lanthanum to Lutetium.
Lanthanides contraction is a phenomenon in chemistry , where there is a decrease in the ionic/atomic radii with the increase in the atomic number due to poor shielding of f-orbitals.
In Lanthanide contraction , the 4 f orbital is not shielded by 5s and 5p orbital , as a result with the increase in the nuclear charge  there is decrease in the atomic size .
So , the elements having same size are Zr-Hf, Mo-W, Nb-T etc.

Final Answer :-
Hence , Option (B) is correct