Which formula represents copper (I) oxide?

(1) CuO (3) Cu2O

(2) CuO2 (4) Cu2O2

The answer is (3) Cu2O. Copper (I) has an oxidation state of + 1 (that's what the "I" indicates). You can also think of this as copper (I) having a charge of + 1. Oxygen has an oxidation state of - 2 (that's just a rule you have to know), and you can think of it as oxygen having a charge of - 2. You need oxidation numbers in a neutral compound to add up to 0 (or charges in a neutral compond to add up to 0), so you need two Cu to balance the O, which is Cu2O.