How many gram atoms are present in $256g$ of $O_2$?

The correct option is A $16$ gram atoms
Given that,
Mass of $O_2$ = 256 g

We know that,
Number of moles = $\left[Mass/Molarmass\right]$

So,

Moles of $O_2$ = $\frac{256}{32}=8$

Number of gram atoms = Number of moles$\ast$Numer of atoms

= $8\times 2=16$ gram atoms

Thus, option A is the correct answer.