Answer:
Correct answer is D. were enslaved states .
Step-by-step explanation:
A is not correct because all of them were states since the beginning of war, including West Virginia that became state when Virginia seceded from the Union.
B is not correct as this were the border states that stayed within the Union.
C is not correct as they were actually the factor that was decisive for the outcome of the war.
D is correct as although they were part of Union, they didn't not prohibited slavery, and Lincoln was cautious with that question regarding their role.