We need the increasing order of ionic radii of $\mathrm{Ce^{3+},\ La^{3+},\ Pm^{3+},\ Yb^{3+}}.$

These are all lanthanoid ions in the common oxidation state $+3$.

Across the lanthanoid series, the ionic radii of $\mathrm{Ln^{3+}}$ ions decrease from La to Lu. This is due to lanthanoid contraction.

So, as atomic number increases: $\mathrm{La^{3+} > Ce^{3+} > Pm^{3+} > Yb^{3+}}$ in size.

Therefore, in increasing order of ionic radius (smallest to largest), we reverse this: $\mathrm{Yb^{3+} < Pm^{3+} < Ce^{3+} < La^{3+}}$

Now compare with the options:

• A: wrong, it is decreasing order reversed incorrectly.
• B: $\mathrm{Yb^{3+} < Pm^{3+} < Ce^{3+} < La^{3+}}$ ✓
• C: wrong, $\mathrm{La^{3+}}$ and $\mathrm{Ce^{3+}}$ are not equal.
• D: wrong, $\mathrm{Ce^{3+}}$ is not larger than $\mathrm{La^{3+}}$.

Hence, the correct option is B.