The type of conflict shown in this excerpt is character vs. character (C).
In The Call of the Wild, different kinds of conflict are at the center of the narration; chiefly character vs. self and character vs. nature.
Yet here, the excerpt is about a confrontation between the protagonist, the dog, and a human he does not know. We are told at the beginning of the text that Buck is not usually hostile towards humans: "he had learned to trust in men he knew." Indeed, Buck is originally a domesticated dog used to living comfortably with humans. What makes this interaction antagonistic is the fact that the man trying to overpower him, and his behavior, are unfamiliar ("the stranger", "surprise").