b
The main reason of lanthanide contraction is, poor shielding of \(4f\) electrons on the outer most shell electrons. As \(f\) orbital is defused nature so shielding of \(4\)f electron is imperfect that is why nucleus can attract outer most shell electron easily and the size of atom decreases.

In lanthanides, there is poorer shielding of \(5 d\) electrons by \(4f\) electrons resulting in greater attraction of the nucleus over \(5d\) electrons and reduction of the atomic radii.