Hi, I may not be too sure about this but I think you have to see it this way.
6,02x10²³ is the Avogrado Constant equivalent to 1 mole of an atom
So when you look at the periodic table or on google for these 3 elements, chlorine gas, Cl₂ and oxygen, O₂ gas are diatomic particles so they exist with 2 of the same element to be stable. So they have 2 times the Avogrado Constant.
On the other hand, Helium gas , He is an inert gas which means it is stable when it exist alone. Therefore 1 mol of helium gas.
Plz correct me if I’m mistaken. 
Hello, 
I think the right answer 6,02x10²³ is the Avogrado Constant equivalent to 1 mole of an atom
I hope this will help you.