Final answer:
The central theme of The Call of the Wild by Jack London is the cruelty of humans towards animals, as represented by Buck's harsh treatment and exploitation for labor in the Canadian wilderness.
Step-by-step explanation:
The central idea or theme about humans' treatment of animals conveyed in The Call of the Wild is most aligned with the cruelty of humans towards animals. Throughout the novel, Jack London explores the brutal conditions and harsh treatment that Buck, the protagonist dog, endures after being sold into sled dog servitude in the Canadian wilderness. The harsh treatment Buck faces represents a broader commentary on the often cruel relationships humans have with animals, especially in the context of the exploitation for labor and the disregard for their wellbeing in the face of human greed and ambition. Buck's narrative highlights a struggle against this cruelty and a return to his innate animal instincts for survival.