This is a key event in The Call of the Wild.

Thornton knelt down by Buck’s side. He took his head in his two hands and rested cheek on cheek. He did not playfully shake him, as was his wont, or murmur soft love curses; but he whispered in his ear. "As you love me, Buck. As you love me,” was what he whispered. Buck whined with suppressed eagerness.

Where does this event fit in the plot’s structure?

rising action
resolution
climax
falling action

This event is part of the falling action in the plot's structure. It is where Thornton pleads with Buck to stay loyal and not give in to the call of the wild, ultimately leading to Buck's internal struggle and eventual decision.