The radius of a proton at the centre is 1.5x10-5 m while the radius of H-atom is 5.32x10-11 m. The ratio of radius of H-atom to the radius of proton is:

Ratio = Radius of H-atom  Radius of Proton =5.32×10−11m1.5×10−5m3.546×10−6. 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 i.e., in 1.5×10−5. So, the correct answer is 3.5×10−6 i.e.,