What is an unreliable narrator? What makes him/her unreliable?

In a group discussion, which statement most clearly uses evidence to support a conclusion? a) the author makes some good points, but i feel that he’s ultimately biased. b) the author proves that he’s biased when he uses terms like “silly” and “stupid”. c) the author obviously hates the way fast food tastes, which is why he’s negative. d) the author first criticizes fast food, and then goes in to criticize junk food.