The effect of lanthanoid contraction in the lanthanoid series of elements by and large means:

First, recall the idea of the effective nuclear charge, generally written as $$Z_{\text{eff}}$$. We have the qualitative relation

$$r \;\propto\; \frac{1}{Z_{\text{eff}}},$$

which tells us that, when the effective nuclear charge felt by the outer electrons increases, the size of the electron cloud—and therefore both the atomic radius and the ionic radius—decreases.

Now, in the lanthanoid (4f-block) elements, electrons are being added successively to the 4f subshell while the principal quantum number remains the same (the valence shell is still 6s2). Because 4f electrons are very poor at shielding one another from the nucleus, the net shielding does not rise in step with the nuclear charge $$Z$$. Hence, as we go from La (Z = 57) to Lu (Z = 71), the quantity $$Z_{\text{eff}}$$ steadily increases.

Substituting this steadily increasing $$Z_{\text{eff}}$$ into the proportionality relation written earlier, we get

$$\text{Higher }Z_{\text{eff}} \;\Longrightarrow\; \text{smaller }r_{\text{atom}} \text{ and smaller }r_{\text{ion}}.$$

So, the phenomenon called “lanthanoid contraction” literally refers to the contraction—that is, the progressive decrease—in both the atomic radii and the ionic radii of the elements as one traverses the lanthanoid series.

Looking at the given choices, the description that matches this conclusion is “decrease in both atomic and ionic radii.”

Hence, the correct answer is Option D.