Final answer:
Abraham Lincoln's tone in the letter was bold but concerned as he expressed determination to settle the slavery question while acknowledging the risks involved.
Step-by-step explanation:
The words that best describe Abraham Lincoln's tone in the letter would be "bold but concerned." Throughout the letter, Lincoln expresses a strong conviction and determination to settle the slavery question, but also shows concern for the consequences of the decision. For example, he states, "We shall nobly save, or meanly lose, the last best hope of earth." This demonstrates his boldness to take action while acknowledging the potential risks involved.
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