According to Bohr's model, the radius of the $n$-th orbit of a hydrogen atom is proportional to $n^2$. If the radius of the first Bohr orbit is $r_1$, the radius of the third orbit is:

**Bohr radius scaling.** $r_n = n^2 a_0$, so $r_3 = 9 a_0 = 9 r_1$. The $n^2$ scaling comes directly from the quantization of angular momentum ($mvr = n\hbar$) combined with the Coulomb force balance $\dfrac{mv^2}{r} = \dfrac{ke^2}{r^2}$. Eliminating $v$ gives $r \propto n^2$. **Why option A ($3 r_1$) is the trap.** A linear scaling matches everyday intuition (3rd shell → 3× radius). Bohr's model — and the Coulomb potential — punish that intuition.