Which of the following contains the greatest number of oxygen atoms?

The correct option is D all have the same no of atoms.
Since the given mass is 1 g of each, the number of oxygen atoms = given mass $÷$ molecular mass $\times N_A$
(i) Given mass of O = 1g, Molar mass of O = 16g
No. of $O$ atom in 1 $g$ =$\frac{1}{16}\times N_A$
(ii) Given mass of $O_2$ = 1g, Molar mass of $O_2$ = 32g
Since, no. of $O_2$ molecule in 1 $g$ =$\frac{1}{32}\times N_A$
Therefore, no. of $O$ atom in 1 $g$ of $O_2$ =$2\times \frac{1}{32}\times N_A$=$\frac{1}{16}\times N_A$
(iii) Given mass of $O_3$ = 1g, Molar mass of $O_3$ = 48g
Similarly, no. of $O$ atom in 1 $g$ of $O_3$ =$3\times \frac{1}{48}\times N_A$=$\frac{1}{16}\times N_A$

All three option has

$\frac{1}{16}moles,henceifnumberofmolesareequalthennumberofoxygenatomsarealsoequal.$

$Threeand2aremutipliedconsequentlyto16asitis{O}_{2}and{O}_3$