Final answer:
President Abraham Lincoln's justification for the Civil War was based on his oath to defend and preserve the Union.
Step-by-step explanation:
President Abraham Lincoln's justification for the Civil War was primarily based on his oath of office to defend and preserve the Union (option C). When the Southern states seceded after Lincoln's election, he believed that their actions were unconstitutional and posed a threat to the unity of the United States. Lincoln's goal in fighting the Civil War was to bring the seceded states back into the Union, rather than to end slavery immediately. Although Lincoln ultimately issued the Emancipation Proclamation in 1863, his initial focus was on maintaining the Union.
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