The best evidence from part 1 of The Call of the Wild that supports the viewpoint that Buck must constantly learn new lessons if he wishes to survive in his new life is:
D. The scene often came back to Buck to trouble him in his sleep. So that was the way. No fair play. Once down, that was the end of you. Well, he would see to it that he never went down.
This passage illustrates Buck's realization about the harsh realities of his new environment, highlighting the importance of learning from experiences to adapt and survive. The acknowledgment that there is "no fair play" and his determination to ensure he "never went down" clearly reflect the lessons he must learn in order to navigate the challenges he faces in the wilderness.