Which of the following quantities contains the greatest number of particles?

2 moles of carbon atoms

2 moles of carbon dioxide molecules

2 moles of diatomic oxygen molecules

all of these quantities contain the same number of particles.

I believe the correct answer would be the last option. All of the quantities given above contain the same number of particles. We determine this by using the avogadro's number. It represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. 

2 moles of carbon atoms ( 6.022 x 10^23 particles / mole  ) = 1.20 x10^24 particles

2 moles of carbon dioxide molecules ( 6.022 x 10^23 particles / mole  ) = 1.20 x10^24 particles

2 moles of diatomic oxygen molecules ( 6.022 x 10^23 particles / mole  ) = 1.20 x10^24 particles

As you can see, no matter what is the gas as long as they have the same number of moles, they would also have same number of particles