Question

How many atoms are in a sample of 175 g of sodium (Na)?

Answer 1

Stoichiometry! 
You'll have to take the 175g of Na and convert it to moles if you want to turn this amount into atoms. Thus, you will have to divide 175g by the molar mass of Na that can be found on the periodic table, which can be rounded to 22.99g. That should give you 7.61 mols. However, since the question is asking for atoms, you'll have to bring in Avogadro's number of 6.022*10^23.
So by taking the 7.61 mols, you'll have to multiply that by 6.022*10^23 in order to get 4.58*10^24 atoms which is B.

A simplified version of all that ^ is:
175g (6.022*10^23) / 22.99g = 4.58*10^24.