And closely akin to the visions of the hairy man was the call still sounding in the depths of the forest. It filled him with a great unrest and strange desires. It caused him to feel a vague, sweet gladness, and he was aware of wild yearnings and stirrings for he knew not what. Sometimes he pursued the call into the forest, looking for it as though it were a tangible thing, barking softly or defiantly, as the mood might dictate. —The Call of the Wild,
Jack London

Think about the title of the novel and how it connects to this excerpt. What does the call stand for?

Thornton’s way of communicating
Buck’s natural instincts
Buck’s desire to be a leader
the horn used to call in the dogs