In The Call of the Wild, the bond between Buck and Thornton is best explained by observing that

ABuck had helped Thornton pay off his debts.
BBuck found Thornton fair and just; Thornton found Buck dependable.
Cdog and man were both in touch with their primordial nature.
Ddog and man were like mirror opposites.

Verified Answer :

Vuk15

in the call of the wild, bond between buck and thornton is best explained by : C. dog and man were both in touch with their primordial nature

for example, in the book we can see that both the dog and the man experience similar experience , the dogs had to be in competition with other dogs and both men and dogs strive to be the leader of the groups

Cdog and man were both in touch with their primordial nature.
Just took the rest