Option B seems to be the best.
African Americans were encouraged to enter politics . Where does the poster mention this?
African Americans were asked to purchase war bonds to support the war . You can clearly see the poster say, "BUY WAR BONDS."
African Americans were only involved in the war effort as nurses . NO, matter of fact they did a lot more than that.
African Americans served on the frontlines and stood as an example of bravery. The poster shows the Air force and the Navy uniforms. Although this might seem correct it says nothing about African Americans serving on the front lines or bravery.
I hope this helps! :)