Once Buck becomes the lead sled dog, the team ...

a.

begins to lag behind.

c.

improves in energy and efficiency.

b.

fights more.

d.

revolts against Perrault and Francois.

Improves in energy and efficiency.

Explanation:

In "The Call of the Wild" by Jack London once Buck becomes the lead sled dog, the team improves in energy and efficiency. Buck has beaten Spitz. Buck thinks that the leadership belongs to him. He takes Spitz place as leader. He appears to be better than Spitz and acts as a born leader. He even manages to break in the two native huskies that were added to the team. They reach Skaguay in record time. The other options are all incorrect, he doesn't revolt against Perrault and Francois, he doesn't fight more and he doesn't begin to lag behind.