Lanthanoid contraction is caused due to which of the following?

The imperfect shielding on outer electrons by 4f electrons from the nuclear charge.

Cause of Lanthanide Contraction:

The effect of lanthanide contraction results from the poor shielding of nuclear charge (with the attractive nuclear force on electrons) by 4f electrons; the 6s electrons can be drawn towards the nucleus, hence resulting in a smaller atomic radius.