The correct answer is D) the American Civil War was caused not by slavery but by economic factors.
An example of a Tesis that a historian might right is "the American Civil War was caused not by slavery but by economic factors."
When writing a Tesis, a historian knows that he has to establish a good and proper hypothesis. So the topic "the American Civil War was caused not by slavery but by economic factors," is a good one because it reveals something different to what is the common beliefe and the historian has to prove it through his research and arguments.