A. 0.5 mol lead (pb) atoms Let's look at the options and see how much they each weigh. A. 0.5 mol lead (pb) atoms * Lead is pretty heavy with an atomic weight of over 200, so this would be a bit over 100 grams. B. 1 mole of hydrogen (H) atoms * Twice as many atoms, but hydrogen only weighs about 1 gram per mole. So A is still has the most mass. C. 1 atom of silver (Ag) * Gotta laugh at this. 1 atom? I don't care how heavy it is, it's gonna have a hard time with about 10^23 lead atoms. So A still has the most mass. D. 25 atoms of iron (Fe) * With that factor of 10^23 going for lead because we're using a half mole, 25 atoms of iron just won't cut it. So of the available choices, we have about 100 grams of lead, about 1 gram of hydrogen. And an unmeasurably small amount of silver and iron. Lead wins easily.