What significant action did Abraham Lincoln take during his presidency?

During his presidency, Abraham Lincoln took the significant action of freeing the slaves through the Emancipation Proclamation in 1863. This landmark executive order declared that all slaves in Confederate-held territory were to be set free, which was a crucial step in the fight against slavery and influenced the eventual passage of the Thirteenth Amendment, which abolished slavery in the United States.

The Emancipation Proclamation not only aimed to weaken the Confederacy but also transformed the Civil War into a battle for freedom. It garnered support for the Union cause among enslaved people and abolitionists, and signaled a moral shift in the war.

In contrast, the other options represent actions or initiatives that do not align with Lincoln's presidency. The National Park Service was established in 1916, well after Lincoln's time. The Declaration of Independence was signed in 1776, long before Lincoln was born. Social Security, as a federal program to support retired workers, was created in the 1930s as part of the New Deal under President Franklin D. Roosevelt. Lincoln's legacy is most strongly tied to his commitment to ending slavery, making the correct answer clear.