Answer
A
Work Step by Step
Molar mass of $O_2: 31.998\ g/mol$, Avogadro's number: $6.022\times10^{23}\ molecules/mol$
In 16 g of $O_2$:
$16\ g\div31.998\ g/mol\times6.022\times10^{23}\ molecules/mol=3.011\times10^{23} molecules$
2 atoms of O/molecule of $O_2$
$6.022\times10^{23}\ atoms$ of O.