Molar mass of O = 16g/(mol)
Mass of O = 16g
Amount of O moles = 16/16 = 1mol
By Avogadro law 1mol of a substance contain `6 xx 10^23` of molecules.
So 1mol of O will contain `6 xx 10^23` molecules.
So the answer is `6xx10^23` molecules. Correct answer is C.