To solve this problem, we need to understand why Zr (Z=40) and Hf (Z=72) have similar atomic and ionic radii.$\newline$Both zirconium (Zr) and hafnium (Hf) belong to the same group in the periodic table, which is the transition metals group.$\newline$However, the reason for their similar atomic and ionic radii is not just because they belong to the same group.$\newline$The key reason is the phenomenon known as the lanthanoid contraction.$\newline$• Lanthanoid contraction refers to the steady decrease in the size of the lanthanide ions (from La to Lu) due to poor shielding of the 4f electrons.$\newline$• This contraction affects the elements that follow the lanthanides, including hafnium.$\newline$• As a result, hafnium has a similar atomic and ionic radius to zirconium, despite having more protons and electrons.$\newline$Analyzing the options:$\newline$• Option 1: Belonging to the same group is true, but it is not the primary reason for the similarity in radii.$\newline$• Option 2: Diagonal relationship is not applicable here.$\newline$• Option 3: Lanthanoid contraction is the correct reason.$\newline$• Option 4: Having similar chemical properties is true, but it does not explain the similarity in radii.$\newline$Therefore, the correct option is Option 3: Lanthanoid contraction.