The reduction in atomic size with increase in atomic number is a characteristic of elements of-

The correct option/answer is f−block.

The reduction in atomic size (or ionic radius) with an increase in atomic number is a prominent and characteristic feature of the elements belonging to the f-block of the periodic table. This phenomenon is famously known as the lanthanide contraction (for the $4f$ series) and actinide contraction (for the $5f$ series).

To understand why this occurs, let's break down the underlying chemistry principles step-by-step:

1. Electronic Configuration:
In f-block elements, the differentiating electrons enter the inner $(n-2)f$ orbitals. For example, in the lanthanide series, the electrons are progressively filled in the $4f$ subshell as the atomic number increases from 58 (Cerium) to 71 (Lutetium).

2. Imperfect Shielding Effect:
The shape and spatial distribution of f-orbitals are highly diffused. Because of this diffused shape, the shielding (or screening) effect of $f$-electrons on the outermost electrons is extremely poor compared to $s$, $p$, or $d$ orbital electrons.

3. Increase in Effective Nuclear Charge:
As the atomic number increases by one unit at each step along the series, the nuclear charge increases by $+1$ due to the addition of a proton. Since the corresponding electron added to the $f$-subshell cannot shield the outer electrons effectively from this rising nuclear charge, the overall effective nuclear charge ($Z_{\text{eff}}$) experienced by the outer shell electrons increases steadily.

4. Contraction in Size:
Because the outer electrons feel a stronger pull from the nucleus, the electron cloud is drawn closer to the nucleus. This results in a continuous and steady decrease in atomic and ionic radii across the f-block series, making this size reduction a defining characteristic of f-block elements.