Which of the following species can act as the strongest base?

Dilution affects both conductivity as well as molar conductivity. Effect of dilution on both is as follows:

Van't Hoff factor for $\mathrm{Na}_2 \mathrm{SO}_4 \cdot 10 \mathrm{H}_2 \mathrm{O}$ solution, assuming complete ionization is

Assertion (A): Zr and Hf are of almost similar atomic radii.

Reason (R): This is due to lanthanoid contraction.