How many molecules of atoms of oxygen are present in 5.6 litres of oxygen $(O_2)$ at NTP?

Question

$\begin{array}{1 1}(a)\;3.01\times 10^{23}atoms&(b)\;2\times 10^{23}atoms\\(c)\;1.9\times 10^{23}atoms&(d)\;4\times 10^{23}atoms\end{array}$