What did John Thornton claim Buck could do? in chapter 5
John Thornton claimed that Buck would risk his own life on John Thornton's demand. He demonstrated this when John signaled to Buck to jump into the river. By doing so, Buck risked his own life. Buck so loved and trusted Thornton, that he knew that his master would never ask him to do anything he wasn't capable of doing.
On another occasion, John Thornton bet money that Buck could pull a sled weighing one thousand pounds. On his command, Buck did so earning the respect of those in attendance and the enduring love from his master.