In the reaction \( \text{MgO} + \text{Cu} \rightarrow \text{Mg} + \text{CuO} \), we need to determine which substance is oxidized.
In this reaction:
- Magnesium oxide (MgO) contains magnesium in the +2 oxidation state and oxygen in the -2 oxidation state.
- Copper (Cu) is in its elemental form (0 oxidation state) and is reduced when it gains electrons.
- Magnesium (Mg) is produced from magnesium oxide, meaning magnesium is reduced in this case (as it goes from +2 in MgO to 0 in elemental Mg).
However, for copper (Cu) to form copper oxide (CuO), it must have been oxidized (it goes from 0 to +2 oxidation state):
- Copper (Cu) loses electrons during this process.
Thus, the correct answer is:
Cu, because it lost electrons.