Which of the following contains the least number of atoms?

Each mole of Ag contains 1 mole of Ag.
No. of moles of atoms in 12 moles of Ag = (12 mol) × 1 = 12 mol

Each mole of H₂O contains 3 moles of atoms (2 moles of H atoms and 1 mole of O atoms).
No. of moles of atoms in 3 moles of H₂O = (3 mol) × 3 = 9 mol

Each mole of N₂O₅ contains 7 moles of atoms (2 moles of N atoms and 5 moles of O atoms).
No. of moles of atoms in 3 moles of N₂O₅ = (3 mol) × 7 = 21 mol

Each mole of C₆H₁₄ contains 20 moles of atoms (6 moles of C atoms and 14 moles of H atoms).
No. of moles of atoms in 1 mole of C₆H₁₄ = (20 mol) × 1 = 20 mol

Hence, 3 moles of H₂O has the least number of atoms.