Total atoms = moles × atoms per formula unit

Steps:

• Total atoms = moles of substance × atoms per molecule × Avogadro's number (compare in equivalent moles of atoms).
• A: 0.5 mol H₂O × 3 atoms = 1.5 mol atoms.
• B: 1.0 mol CO₂ × 3 atoms = 3.0 mol atoms.
• C: 1.0 mol CH₄ × 5 atoms = 5.0 mol atoms.
• D: 2.0 mol HCl × 2 atoms = 4.0 mol atoms.

Why C is correct:

• Methane (CH₄) has 5 atoms per molecule, so 1 mol yields 5 mol of atoms by Avogadro's number.

Why the others are wrong:

• A: 0.5 mol × 3 atoms/molecule = only 1.5 mol atoms.
• B: 1 mol × 3 atoms/molecule = 3 mol atoms.
• D: 2 mol × 2 atoms/molecule = 4 mol atoms.

Final answer: C