Final answer:
The moral cause the Emancipation Proclamation gave to the Union was the abolition of slavery, enhancing the Union's military strategy and complicating international support for the Confederacy. Additionally, it allowed the recruitment of African American soldiers, enhancing the Union's manpower while adding a profound moral dimension to the war effort.
Step-by-step explanation:
The moral cause that the Emancipation Proclamation gave to the Union during the Civil War was the abolition of slavery. The Proclamation, issued by President Abraham Lincoln, fundamentally changed the nature of the war because it officially set the Union against the institution of slavery and committed to its eradication as a war aim. Thus, the correct answer to the student's question is 1) Abolition of slavery. Factoring into this decision were several important considerations including military strategy and international diplomacy; the Proclamation undermined the Confederacy's war effort by announcing the freedom of slaves in the rebellious states and it discouraged European powers, especially Britain, from recognizing or supporting the Confederacy because of the issue of slavery and the British anti-slavery sentiment.
Furthermore, by allowing for the recruitment of African American soldiers into the Union army, it enhanced the Union's manpower. It has to be noted that while the Emancipation Proclamation was a crucial step towards ending slavery, it did not immediately free all slaves in America; it specifically targeted regions under Confederate control and left alone those in the Border States and other regions already under Union control. Despite its limitations, the Proclamation added a profound moral impetus to the Union cause by aligning it with the fight against slavery.