The graphical representation of number of molecules of different gases is given. Which gases are placed at correct position ?

The number of moles is given by $\mathrm{Number} \mathrm{of} \mathrm{moles} \left(\mathrm{n}\right)= \frac{\mathrm{Mass}}{\mathrm{Molecular} \mathrm{mass} }$.

Number of molecules=$\mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=\mathrm{n}\times 6.023\times {10}^{23}$

For ${\mathrm{H}}_2,$$\mathrm{n}=\frac{1}{2}\times 6.023\times {10}^{23}= 3.01\times {10}^{23}$ which do not match with graphical value.

For $\mathrm{He},$$\mathrm{n}=\frac{2}{4}\times 6.023\times {10}^{23}= 3.01\times {10}^{23}$ which matches with graphical value.

For ${\mathrm{N}}_2,$$\mathrm{n}=\frac{3}{28}\times 6.023\times {10}^{23}= 0.069\times {10}^{23}$ which do not match with graphical value.

For ${\mathrm{CH}}_4,$$\mathrm{n}=\frac{4}{16}\times 6.023\times {10}^{23}= 1.5\times {10}^{23}$ which matches with graphical value.

$He,C{H}_4$  are placed correcly in the graph.