Which must be the same when comparing 1 mol of oxygen gas, o2, with 1 mol of carbon monoxide gas, co? a. the mass b. the volume c. the number of molecules d. the number of oxygen atoms

A mole is a rather unusual unit of measurement. It is defined by a specific number of particles, molecules, atoms, or any other unit. 
That number happens to be 6.022×10²³. 
When we talk about a mole of water, then, we refer to 6.022×10²³ molecules of water. 

Comparing 1 mole of 02 with 1 mole of CO:
This means means that both quantities will have the same number of molecules. The answer is C.

It may be tempting to select D, since it relates to the number of atoms rather than mass or volume, and moles are related to this idea. However, keep in mind that one mole of O2 will actually have twice as much oxygen atoms as one mole of CO, since there are two oxygen atoms per O2 molecule and only 1 oxygen atom per CO molecule.