The correct answer is D. The Union cause.
Sullivan Ballou (1829-1861) was an American military and politician who volunteered in the Civil War on behalf of the Union. His popularity is due to the letter he wrote to his wife Sarah because in this letter he expressed his feelings about the Civil War and the cause of the Union "I have no misgivings about or lack of confidence in the cause in which I am engaged, and my courage does not halt or falter ... And I am willing - perfectly willing - to lay down all my joys in this life, to help maintain this Government, and to pay that debt ". According to the above, it can be stated that the correct answer is D. The Union cause.