The maximum number of molecules is present in:-

The number of molecules is proportional to the number of moles, so compare the moles. At STP, 1 mole of gas occupies 22.4 L.
5 L of $N_2$ at STP: $\dfrac{5}{22.4} = 0.223$ mol
0.5 g of $H_2$: $\dfrac{0.5}{2} = 0.25$ mol
10 g of $O_2$: $\dfrac{10}{32} = 0.3125$ mol
15 L of $H_2$ at STP: $\dfrac{15}{22.4} = 0.67$ mol
The largest number of moles is in 15 L of $H_2$, which contains $0.67 \times 6.022 \times 10^{23} \approx 4.0 \times 10^{23}$ molecules.
So the maximum number of molecules is present in 15 L of $H_2$ gas at STP.