Answer: Confederate
Explanation:
Confederate is someone who participate under a secret or illegal activity.
Milgram experiment was conducted by Stanley Milgram a psychologist who wanted to find out how people abide to obedience. His focus was on how people deal with obedience to authority and separating that from their own conscience.
There would be a learner and a teacher in his experiment the learner is what he called Confederate who will act as participants.
So in the experiment the learner will be exposed to the extreme conditions that would be put on someone who has to obey and see how they would react and then he would be able to draw his conclusion on obedience and conscience.

