Answer:
After Spitz is killed, Buck takes the place of the lead dog by refusing to be harnessed unless he is at the front of the line.
Step-by-step explanation:
Buck has a leadership conflict with Spitz, and after an intense fight for power between Buck and Spitz in chapter three where Spitz gets killed, Buck supposes that he has won the leader position, then when Sol-leks is put in that place, he refuses to be harnessed and after a while trying to get him, they decided to put him as a leader to keep on working.