The correct option is A 2N2O(g)
The expression that can be used to designate 6 moles of atoms is 2N2O(g).
It corresponds to two moles of N2O(g) molecules or 6 moles of atoms.
The expressions H2(g) , CO2(g) , 2NaCl(aq), H2SO4 (dilute aq) designate 2 moles, 3 moles, 4 moles and 7 moles of atoms respectively.