Answer
$B\lt C \lt A$
Work Step by Step
We know that Bohr radius is proportional to $\frac{n^2}{Z}$.
For atom (A) $\frac{n^2}{Z}=\frac{2^2}{1}=4$
For atom (B) $\frac{n^2}{Z}=\frac{1^2}{2}=\frac{1}{2}$
For atom (C) $\frac{n^2}{Z}=\frac{3^2}{3}=3$
Thus, the order of increasing Bohr radius is
$B\lt C \lt A$
