The radius of a proton at the centre is $1.5x{10}^{-5}$ m while the radius of H-atom is $5.32x{10}^{-11}$ m. The ratio of radius of H-atom to the radius of proton is:

$\text{ Ratio }=\frac{\text{ Radius of H-atom }}{\text{ Radius of Proton }}=\frac{5.32\times {10}^{-11}\mathrm{m}}{1.5\times {10}^{-5}\mathrm{m}}$

$3.546\times {10}^{-6}$. In division, the number of significant figures in the final answer should not be greater than the number of significant figures in the least precise factor. The least number of significant figures $=2\left(\text{ i.e., in }1.5\times {10}^{-5}\right)$. So, the correct answer is $3.5\times {10}^{-6}$ i.e.,