Explanation

The hydrogen atom has a diameter of about 1 angstrom, i.e. 10⁻¹⁰ metre (the Bohr radius is 0.53 Å). Ten micrometres is the scale of a human cell and ten millimetres is macroscopic — both far too large for an atom. Hence (A) 10⁻¹⁰ (metres) is the correct size.