What is the formula for this compound?
1 atom of copper, 1 atom of sulfur, 4 atoms of oxygen

Answer:
The third one: [tex]CuSO_4[/tex]
Explanation:
Again you are going to want to look at a periodic table so you can determine the symbol of each atom.
The symbol for copper is Cu, the symbol for sulfur is S and the symbol for Oxygen.
There is 1 atom of copper in the compound
So we have Cu
There is 1 atom of sulfur in the compound
So we would have CuS
There are 4 atoms of oxygen in the compound
So we would have [tex]CuSO_4[/tex]
Note that the little number at the bottom left after each symbol indicates how many atoms of that element are in that compound.