Answer: A - Saved (or preserved) the Union.
One of the most important things Abraham Lincoln did was preserve the Union during the American Civil War. As the 16th President of the United States, Lincoln faced Southern states' secession, which led to the Civil War. His leadership during this tumultuous period was crucial in maintaining the unity of the United States. He firmly believed in keeping the country united and worked tirelessly towards this goal.
The other options listed are not associated with Abraham Lincoln. The United Nations was established long after Lincoln's time, in 1945. The United States declared war on Great Britain during the War of 1812, well before Lincoln's presidency. Lastly, the purchase of Alaska, known as "Seward's Folly," was negotiated by Secretary of State William H. Seward in 1867, two years after Lincoln's assassination.
