What is copper oxide reduced to when it reacts with carbon?

2CuO + C CO2(g) + 2Cu

CuO is reduced because the oxidation number of copper decreases from +2 (in CuO) to 0 (in Cu).

C is oxidised because the oxidation number of carbon increases from 0 (in C) to +4 (in CO2).

So this can be considered a redox reaction as both reduction and oxidation occurs at the same time. Since CuO is reduced, it acts as the oxidising agent. Since C is oxidised, it acts as the reducing agent.