Reason of lanthanoid contraction is: negligible screening effect of f orbitals. increasing nuclear charge. decreasing nuclear charge. decreasing screening effect.

Reason of lanthanoid contraction is Negligible screening effect of f orbitals. hence, as the nuclear charge increases, the valence shell is pulled slightly towards the nucleus. Thus the size of the tripositive ion decreases with an increase in the atomic number. This decrease is very small and is known as lanthanide contraction. Hence, the correct option is A.