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
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
.
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.