Step 1: The very name of the effect points to the answer. It is called lanthanide contraction, and the lanthanides (\(_{57}\)La to \(_{71}\)Lu) form the first row of the inner-transition series.
Step 2: Inner-transition elements are placed in the f-block of the periodic table because their differentiating electron occupies an \(f\) orbital.
Step 3: Poor screening by \(4f\) electrons makes each successive nucleus grip its electrons a little tighter, so the radius shrinks smoothly across the series. This shrinking is exactly the lanthanide contraction.
Step 4: Since the phenomenon is defined for and observed in the f-block inner-transition elements, option (iv) is correct.
\[\boxed{\text{f-block element}}\]