Answer: A. Challenging the credibility of sources.
Explanation: In the given conversation between Emily and Han, we can see that Emily clearly shows the group discussion technique of challenging the credibility of sources, by saying that Pettit's assumptions are probably inaccurate because they are based in a source that isn't reliable, the book "Medieval Masterpieces" by Francis Eggbert because even though Eggbert is an historian, that book is fiction, so it isn't a credible source.
