Final answer:
The bombardment of Fort Sumter on April 12, 1861, initiated the Civil War, triggering a call for Union troops by President Lincoln.
Step-by-step explanation:
The event that marked the official beginning of armed hostilities between the North and South in April 1861 was C. The bombardment of Fort Sumter. On April 12, 1861, Confederate forces began firing on the Union garrison at Fort Sumter located in Charleston Harbor, which launched the nation into the Civil War. This assault prompted President Abraham Lincoln to issue a Proclamation calling for 75,000 militiamen to form an army to suppress the rebellion, confirming the start of the war.