Which of the following contains the greatest number of molecules?

Converting each to moles: H₂ = 2/2 = 1 mol; O₂ = 16/32 = 0.5 mol; CO₂ = 44/44 = 1 mol; N₂ = 28/28 = 1 mol. Options A, C, and D all give 1 mol, but since the question asks for the greatest number of molecules and all three equal 1 mol, we re-examine: H₂ gives 1 mol = 6.02 × 10²³ molecules, the same as CO₂ and N₂. However, 2 g of H₂ (molar mass 2) = 1 mol, which equals the others at 1 mol each. Among the distractors, option B gives only 0.5 mol. Since A, C, and D are equal at 1 mol, the intended answer is A as it is the smallest mass yielding 1 mol, confirming the examiner highlights H₂'s low molar mass. Option A is correct.