What was the decisive factor in the North's success in the final years of the war?

A divided leadership

B greater resources

C strong legal arguments

D political unity

Of the answers you listed I would say greater resources, which played a large role in the success of the north. The south had better leaders with more military experience, which is the reason for the south's considerably less amount of casualties. The north also had a much larger population they could draw soldiers from. Political unity is a joke, it's a war between two organized militaries. And you can't have order with divided leadership. I'd say B is the best answer.