Hello Kate, to thwart is to sabotage or prevent something from happening. To confederate means to "join together through an agreement". So the best opposite word is to hew, which means to split.

Btw, in case you don't know yet, the new GRE test does not include vocabulary test anymore. We leave the vocabulary practice exercises inside the app by popular demand, because people feel they are useful to increase the number of English words they know.