The reduction in atomic size with increase in atomic number is a characteristic of elements of-
A d-block
B f-block
C Radioactive series.
D. High atomic masses

Hint: As we know that, when we move left to right in the period, atomic size decreases with increase in nuclear charge of the element. But when we go top to bottom in the group, atomic size increases. The shielding effect is described as the phenomenon by which the inner shell electron shields the outer shell electron so they are not affected by nuclear charge.

Complete step by step answer:
Let’s discuss it in detail.
When the shielding is not as good, this would mean that the positively charged nucleus has a greater attraction to the electrons, thus decreasing the atomic radius as the atomic number increases. The s orbital has the greatest shielding while f has the least and p and d in between the two with p being greater than d.
Elements in which the last electrons enter into f subshell belong to the f-block element and we know that the lanthanide contraction is the result of a poor shielding effect of the 4f electrons.
So the reduction of atomic size with increase in atomic number is a characteristic of elements of f-block.

Therefore, the correct option is option (B).

Note:
When the shielding is not good there will be a greater nuclear charge, thus pulling the electrons is closer, resulting in a smaller than expected radius.