How to identify a redox reaction?
You are confused because the answer key is wrong.
The way to figure it out is to assign oxidation numbers to each species following the rules. Oxidation states are simply a convenient way to keep track of electrons without having to draw out Lewis structures, count electrons pairs and so on. The rules help make that happen.

Notice the position of the sign relative to when you indicate charge. It is not the same.

The correct answer is C though: $$\ce{CuO + CO -&gt; Cu + CO2}$$
with oxidation states 
$$\ce{\overset{(+2)}{Cu}\overset{(-2)}{O} 
    + \overset{(+2)}{C}\overset{(-2)}{O} 
   -&gt; \overset{(0)}{Cu} 
    + \overset{(+4)}{C}\overset{(2\times(-2))}{O2}}$$
Notice Cu went from +2 to 0. It gained 2 electrons to do that, thus it was reduced. What gave up the electrons? The other carbon, because it went from +2 to +4: its oxidation state increased by 2, thus it lost 2 electrons.

Recall OIL RIG: Oxidation is loss of electrons and reduction is gain of electrons. 
Whatever is oxidized increases its oxidation state, and whatever is reduced decreases it.