Which of the following pairs does not have same number of molecules?      

Same number of moles indicates equal number of molecules.

For option (A):

${\mathrm{n}}_{{\mathrm{O}}_2}=\frac{2 \mathrm{g}}{32 \mathrm{g}/\mathrm{mol}}=\frac{1}{16} \mathrm{mol}$,  ${\mathrm{n}}_{{\mathrm{SO}}_2}=\frac{4 \mathrm{g}}{64 \mathrm{g}/\mathrm{mol}}=\frac{1}{16} \mathrm{mol}$

For option (B):

${\mathrm{n}}_{{\mathrm{CO}}_2}=\frac{2 \mathrm{g}}{44 \mathrm{g}/\mathrm{mol}}=\frac{1}{22} \mathrm{mol}$, ${\mathrm{n}}_{{\mathrm{N}}_2\mathrm{O}}=\frac{2 \mathrm{g}}{44 \mathrm{g}/\mathrm{mol}}=\frac{1}{22} \mathrm{mol}$

For option (C):

${\mathrm{n}}_{{\mathrm{O}}_2}=\frac{224 \mathrm{mL}}{22400 \mathrm{mL}}\times 1 \mathrm{mol}=\frac{1}{100} \mathrm{mol}$,  ${\mathrm{n}}_{\mathrm{He}}=\frac{\mathrm{P}\times \mathrm{V}}{\mathrm{R}\times \mathrm{T}}=\frac{0.5\times 0.448}{0.0821\times 273}=\frac{1}{100} \mathrm{mol}$

For option (D):

${\mathrm{n}}_{{\mathrm{O}}_2}=\frac{2 \mathrm{g}}{32 \mathrm{g}/\mathrm{mol}}=\frac{1}{16} \mathrm{mol}$,    ${\mathrm{n}}_{{\mathrm{O}}_3}=\frac{2 \mathrm{g}}{48 \mathrm{g}/\mathrm{mol}}=\frac{1}{24} \mathrm{mol}$

Hence, correct option (D).