Which balanced equation represents a redox reaction?
(1) agno3 + nacl ==> agcl + nano3
(2) bacl2 + k2co3 ==> baco3 + 2kcl
(3) cuo + co==> cu + co2
(4) hcl + koh ==> kcl + h2o

In order to determine which reaction is redox, you need to compare oxidation the oxidation numbers of each element on both sides of the equation (a change in the oxidation number for any of the elements indicates oxidation-reduction). In order to do that, you first need to assign oxidation numbers to each element on both sides of the equation (if you don't know how to do this, you should learn this first). The answer is (3). Cu goes from +2 to 0 and C goes from +2 to +4.