By inspection, it is easy to tell what the answer should be. The first and last options imply that there isn’t a whole atom in the given amount, which is incorrect because you can’t have a fraction of an atom and the given mass of copper is going to have a lot more than one atom in it anyway. The second option is an absurdly large value and can also be ruled out, leaving you with the third option.
For future reference, a more mathematical approach is below:
To find the number of atoms in a given mass of substance, divide the mass by the molar mass, and then multiply by Avogadro’s number.
[tex](68.7 \text{ g Cu})(\frac{1 \text{ mol Cu}}{63.55 \text{ g Cu}} )(\frac{6.022*10^{23} \text{ atoms}}{1 \text{ mol}} )=6.51*10^{23} \text{ atoms Cu}[/tex]
Checking units reveals that there were no errors with units.
