Which of the following compounds has maximum number of molecules ?

Number of molecules is directly proportional to the number of moles.

Number of moles of $\text{H}_2 = \dfrac{2}{2} = 1$ mol

Number of moles of $\text{N}_2 = \dfrac{7}{28} = 0.25$ mol

Number of moles of $\text{O}_2 = \dfrac{8}{32} = 0.25$ mol

Number of moles of $\text{NO}_2 = \dfrac{20}{46} \approx 0.43$ mol

Since $2$ g of $\text{H}_2$ has the maximum number of moles, it contains the maximum number of molecules.

Answer: 2 g of $\text{H}_2$