c
Let us choose the nucleus of the hydrogen atom as the origin for measuring distance. Mass of hydrogen atom, \(m_{1}=1\) unit (say) since cholorine atom is \(35.5\) times as massive as hydrogen atom, \(\therefore\) mass of cholorine atom, \(m_{2}=35.5\) units

Now, \(x_{1}=0\) and \(x_{2}=1.27 \mathrm{A}=1.27 \times 10^{-16} \mathrm{m}\)

Distance of centre of mass of HC1 molecule from the origin is given by 

\(X=\frac{m_{1} x_{1}+m_{2} x_{2}}{m_{1}+m_{2}}=\frac{1 \times 0+35.5 \times 1.27 \times 10^{-10}}{1+35.5} m\)

\(=\frac{35.5 \times 1.27}{36.5} \times 10^{-10} \mathrm{m}=1.235 \times 10^{-10} \mathrm{m}=1.24 \mathrm{A}\)