16 g of oxygen has same number of molecules as in
The molar mass of oxygen is 32. Therefore, 32g of oxygen contains 6.023 x 1023 molecules. 16g of oxygen contains x 6.023 x 1023 = 0.5 x 6.023 x 1023 molecules.
16g of CO contains x 6.023 x 1023 = 0.57 x 6.023 x 1023 molecules.
28g of N2 contains x 6.023 x 1023 = 6.023 x 1023 molecules.
14g of N2 contains x 6.023 x 1023 = 0.5 x 6.023 x 1023 molecules.
1.0g of H2 contains x 6.023 x 1023 = 0.5 x 6.023 x 1023 molecules.
Hence oxygen has the same number of molecules as (iii) and (iv).