Final answer:
President Lincoln hesitated to enlist African Americans due to fears of unpopularity in the border states and threats to the loyalty of white volunteers. However, as the number of escaped slaves seeking refuge increased, Lincoln allowed African Americans to enlist as Union soldiers.
Step-by-step explanation:
President Lincoln hesitated at first to enlist African Americans because he feared that enlisting them would be unpopular in the border states (option A). He was concerned that white volunteers might refuse to serve alongside black soldiers and that it would threaten the loyalty of slaveholding border states. However, as the number of escaped slaves seeking refuge behind Union army lines grew, Lincoln and Congress were forced to reconsider and eventually allowed African Americans to enlist as Union soldiers.