The event that marked the official beginning of the Civil War was:
C. Confederate troops firing on Fort Sumter.
This event took place on April 12, 1861, when Confederate forces in South Carolina attacked the Union-held Fort Sumter. The Confederate bombardment forced the surrender of the fort, leading to the start of the Civil War between the North (Union) and the South (Confederacy). This act of aggression by the Confederates against a Union military installation was a pivotal moment that triggered the outbreak of hostilities and marked the official start of the Civil War.