Answer : Option C) The number of molecules.
Explanation : When comparing 1 mol of oxygen gas with 1 mol of carbon monoxide the number of molecules must be same. Each mole should contains [tex] 6.023 X 10^{23} atoms [/tex] in it. Irrespective of the weight or atomic number avogadro's number remains constant for every mole of atom.
It is a number which is usually given for atoms or molecules, that are contained in the amount of substance given by one mole.
It is denoted by [tex] N_{A} [/tex].
The units of Avogadro's number can be expressed in terms of electrons, atoms, ions, or molecules, depending on the nature of the substance and the character of the reaction.
