What this is asking you is what things there are in the book that show two characters that are opposed to each other in some important ways. These oppositions help to make the points that the author is trying to show us.
I can think of two examples right off that migh help you.
First, there is the pairing of Buck and Spitz. These two dogs have different character traits. Mainly Buck is just and Spitz is not. We see this when he is so amused when Curly is killed. That is why the two of them come in conflict.
Second, there is the contrast between John Thornton and the three people who owned Buck just before Thornton got him. Charles and Hal and Mercedes totally don't care about the dogs. This is directly opposite of what Thornton is like.