The number of atoms =

(a) Number of atoms in 18 g of H₂O = 18/18 × N = N

(b) Number of atoms in 18 g of O₂ = 18/32 × N = 0.562N

(c) Number of atoms in 18 g of CO₂ = 18/44 × N = 0.409 N

(d) Number of atoms in 18 g of CH₄ = 18/16 × N = 1.15 N

As seen, the maximum number of atoms are present in 18g of CH₄