Identify the ions and the trend involved

We need to compare the sizes of: $Bk^{3+} \quad \text{and} \quad Np^{3+}$

Here:

• $Np$ = Neptunium, an early actinide
• $Bk$ = Berkelium, a later actinide

Use the actinoid contraction trend

Across the actinoid series, the ionic radii of ions of the same charge generally decrease with increasing atomic number. This is called actinoid contraction.

Since $Bk$ comes much later than $Np$ in the actinide series, $r(Bk^{3+}) < r(Np^{3+})$

Therefore, Assertion A is true.

Examine the reason

Reason says this is due to lanthanoid contraction.

But the comparison is between two actinoid ions ($Np^{3+}$ and $Bk^{3+}$), so the size decrease is due to actinoid contraction, not lanthanoid contraction.

Hence, Reason R is false.

Choose the correct option

• Assertion A: True
• Reason R: False

So the correct option is: $\boxed{\text{C}}$

Compare with stored correct answer

Stored correct answer: C

My derived answer matches it.