The maximum number of molecules is present in :

In 15 L of $H_2$ gas at STP,
the number of molecules $=\frac{6.023\times {10}^{23}}{22.4}\times 15$
$=4033\times {10}^{23}$
In 5 L of $N_2$ gas at STP, ltbr. the number of molecules $=\frac{6.023\times {10}^{23}\times 5}{22.4}$
$=1.344\times {10}^{23}$
In 0.5 g of $H_2$ gas,
the number of molecules $=\frac{6.23\times {10}^{23}\times 0.5}{2}$
$=1.505\times {10}^{23}$
In 10 g of $O_2$ gas ,
the number of molecules $=\frac{6.23\times {10}^{23}\times 10}{32}$
$=1.882\times {10}^{23}$
Hence , maximum number of molecules are presents in 15 L of $H_2$ at STP.