Reason of lanthanoid contraction is :
Negligible screening effect of 'f' orbitals
Increasing Nuclear charge
Decreasing nuclear charge
Decreasing screening effect
The lanthanoid contraction results from the poor shielding effect of the 4f orbitals due to which outer 6s electrons are more strongly attracted by the nucleus thus resulting in a decrease in size of the atoms subsequent to lanthanoids i.e from hafnium.
The correct answer is option 1.