The correct answer is b. Conflict is the conqueror's life; the only way to get more land is to take it from someone else.
To find the best option lets investigate what a zero-sum game means. It represents that the victory of one side automatically represents the loss of the other. All the other options do not qualify in this definition, given that they represent a win/win game and a logistic example.
The Conqueror's story expressed in the text qualifies exactly for the zero-sum game. Given that the win of the conqueror (land) is based on the losses of the conquered (loss of land).