Answer:
<u>d. a row of elements</u>
Explanation:
Elements that are in the same group, have same number of electrons and make up a column in a periodic table.
The only list containing molecules is list #4
NaCl, N2, and H2O are all made up of more than one element, making them molecules. The other lists contain an element. Mg, He, and Fe are elements, not molecules.
Hope this helps!
I think the answer is A or C sorry for not having that good of an answer