Answer:
The correct answer is C. Napoleon was a great general, but he was also very arrogant.
Step-by-step explanation:
In the sentence there are two contrasting ideas: Napoleon is a great general-He is arrogant.
These two must be joined by a coordinating conjunction that can show the contrastive relationship between them. 'But' is the right choice because it's the one coordinating conjunction that shows contrast when joining independent clauses.