The maximum number of molecules are present in

Since the number of elementary particles in a sample is proportional to the amount of the substance in moles, the sample that contains the maximum amount in moles contains the maximum number of molecules.
$$\begin{gathered} 15\mathrm{L}\text{ of }{\mathrm{H}}_2\text{ at STP}=15\mathrm{L}\times \frac{1\mathrm{mol}}{22.4\mathrm{L}}=0.669 \mathrm{mol}, \\ 5\mathrm{L}\text{ of }{\mathrm{N}}_2\text{ at STP}=5\mathrm{L}\times \frac{1\mathrm{mol}}{22\cdot 4\mathrm{L}}=0.223 \mathrm{mol}, \\ 0.5\mathrm{g}\text{ of }{\mathrm{H}}_2\text{ gas }=0.5\mathrm{g}\times \frac{1\mathrm{mol}}{2\mathrm{g}}=0.25 \mathrm{mol}, \\ 10\mathrm{g}\text{ of }{\mathrm{O}}_2\text{ gas }=10\mathrm{g}\times \frac{1\mathrm{mol}}{32\mathrm{g}}=0.3125 \mathrm{mol}. \end{gathered}$$