Which of the following pairs has the same size?

Zirconium (Zr) belonging to the 4d series is similar in size to Hafnium (Hf) belonging to the 5d series.

This is because of Lanthanoid or Lanthanide contraction.

Lanthanide refers to the steady decrease in size of atoms or ions of rare earth metals with the increase in atomic number.

This is because electrons entering or present in the f-subshell do not provide a good shielding effect with the increase in atomic number or effective nuclear charge.

Final Answer:

The correct option is D