Which of the following factor may be regarded as the main cause of lanthanide contraction?

This phenomenon is associated with the intervention of the 4f orbitals which must be filled before the 5d series of elements begin. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called is lanthanoid contraction. This is  because of poor shielding of one of the 4f-electrons by another in the sub-shell.