Question

Read the excerpt from The Call of the Wild.

Answer 1

The correct answer here is definitely A.

The excerpt shows the wilderness it is dog eat dog world and the only the strongest will survive. This is even explicitly said in the sentence: 

The blood-longing became stronger than ever before. He was a killer, a thing that preyed, living on the things that lived, unaided, alone, by virtue of his own strength and prowess, surviving triumphantly in a hostile environment where only the strong survived.

The wilderness does not care and it is a hostile environment and if you want to survive you have to strong.

Answer 2

The theme would be supported by these lines:The blood-longing became stronger than ever before. He was a killer, a thing that preyed, living on the things that lived, unaided, alone, by virtue of his own strength and prowess, surviving triumphantly in a hostile environment where only the strong survived. 
So I think the answer to your question would be the theme that  illustrates this is that  Only the strong survive in the wilderness.