Final answer:
Irish immigrants supported the Union in the Civil War as they saw the United States as a symbol of freedom, relating their own struggles for Irish independence to the Union's fight against slavery and for democracy.
Step-by-step explanation:
The most likely reason for the Union support by Irish Immigrants during the American Civil War is option c: Irish immigrants viewed the United States as a nation that had achieved freedom from Great Britain and saw this freedom as an example for a future Irish nation free of British rule. Irish immigrants like Brigadier General Thomas Francis Meagher, drew parallels between the American quest for freedom and their own failed efforts to create an Irish Republic.
They supported the Union as a symbol of liberty against what they perceived as the tyrannical British rule that had caused them so much suffering during the Potato Famine. Furthermore, the Union's later stance on emancipation and the fight against slavery resonated with their desire for freedom and equal rights, aligning with abolitionist sentiments that were prevalent after Great Britain's own abolition of slavery in 1833.