Answer: +2
Explanation: Copper has a +2 oxidation number in CuO.
This is due to the fact that oxygen has an oxidation number of 2, and the entire chemical has a neutral charge. Consequently, the following equation can be used to determine copper's oxidation number:
(+2) + (-2) = 0
In order to counteract the -2 oxidation number of oxygen in CuO, copper must have an oxidation number of +2.