A metalloid is a type of chemical element which has a preponderance of properties in between, or that are a mixture of, those of metals and nonmetals.
 
        
                    
             
        
        
        
The number of formula units in 2.50 mol of the compound is  15.1 * 10^23. 
The question is unclear whether NaNO2 or NaNO3 is implied. However,in either case, the solution applies equally.
6.02 * 10^23 formula units of the compound are contained in 1 mole
x formula units are contained in 2.5 moles of the compound
x = 6.02 * 10^23 formula units * 2.5 moles/ 1 mole
x = 15.1 * 10^23 formula units of the compound.
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An anion is an ion with a negative charge. The minus sign, when attached to the end of an ionic compound, indicates that this has a negative charge, while a plus sign indicates a positive charge. 
O2- is the only compound listed that satisfied this. It is the anion.
Answer is 02-