Which sample has the most number of atoms? A. "1 mole of oxygen atoms;" B. "1 mole of oxygen molecules;" C. "1 mole of ozone molecules;" D. "Each sample contains the same number of atoms."

Surely it is option $C .$?
Mass is a measure of the number of particles. Here you have $O$ atoms, and dioxygen, ${O}_{2}$, and ozone molecules, ${O}_{3}$ but each have the same mass by specification. There are thus equal numbers of atoms, massive particles.