The correct answer is:
(D) Lincoln's tone in his letter and the mother's tone in The Red Badge of Courage are similar.
Both passages address the theme of the sacrifices of war and convey a similar tone of solemnity and grief. Lincoln's letter is compassionate and empathetic towards the sacrifices made by soldiers and their families, while the mother's tone in The Red Badge of Courage is humble and resigned, reflecting the pain and hardship that families of soldiers had to endure. Therefore, the most appropriate choice is (D).