a
\(4\,g\) of hydrogen \(= 4\) mole of hydrogen

                               \(= 4 \times 6.023 \times 10^{23}\) atoms 

\(71.0\,gm\) of chlorine \( = \frac{{71.0}}{{71.0}} = 1\) moles of chlorine

                                    \(= 6.023 \times 10^{23}\) atoms

\(127\,gm\) of iodine \( = \frac{1}{2}\) mole of \(I_2\)

\( = \frac{{127}}{{254}} = 6.023 \times {10^{23}} \times \frac{1}{2}\) atoms

\(48.0\,gm\) of magnesium

\( = \frac{{48.0}}{{24.0}} = 2 \times 6.023 \times {10^{23}}\) atoms

\(\therefore \) \(4.0\,gm\,H_2\) has largest number of atoms