4g of hydrogen = 4 mole of hydrogen
= $4 \times 6.023 \times 10^{23}\, atoms$
$71.0 \,g$ of chlorine = $\frac{71.0}{71.0}$ = $1$ mole of
chlorine = $6.023 \times 10^{23}$ atoms.
$127\, g$ of iodine = $\frac{1}{2}$ mole of $I_2$
$=\frac{127}{254}=6.023\times10^{23}\times\frac{1}{2}$ atoms.
$48.0\, g$ of magnesium
$=\frac{48.0}{24.0}=2\times6.023\times10^{23}$ atoms.
$\therefore 4.0\, g \,H_2$ has largest number of atoms.