Number of molecules in $15L$ of $H_2=\large\frac{15}{22.4}$$\times N$

$\qquad\qquad\qquad\qquad\quad\quad\;\;\;\;\;=0.669N$

Number of molecules in $5L$ of $N_2=\large\frac{5}{22.4}$$\times N$

$\qquad\qquad\qquad\qquad\quad\quad\;\;\;=0.223N$

Number of molecules in 0.5g $H_2=\large\frac{0.5}{2}$$\times N$

$\qquad\qquad\qquad\qquad\quad\qquad\;\;=0.25N$

Number of molecules in 10g $O_2=\large\frac{10}{32}$$\times N$

$\qquad\qquad\qquad\qquad\quad\qquad\;=0.312N$

Hence (a) is the correct answer.