2. F-block.

Explanation:
A reduction in atomic size with increase in atomic number is a characteristic of f-block elements.
This is due to poor shielding of f electrons. The extent of actinoid contraction is greater than lanthanoid contraction. With increase in atomic number i.e. in moving down a group, the number of the principal shell increases and therefore, the size of the atom increases. But in case of f ­block elements there is a steady decrease in atomic size with increase in atomic number due to lanthanide contraction. As we move through the lanthanide series, 4f electrons are being added one at each step.
The mutual shielding effect of f electrons is very little. This is due to the shape of the f ­orbitals. The nuclear charge, however increases by one at each step. Hence, the inward pull experienced by the 4f electrons increases. This causes a reduction in the size of the entire 4fn shell.