Final answer:
The last event was the Northern troops marching through Richmond, Virginia, after capturing the city in April 1865, indicating the Civil War's imminent end.
Step-by-step explanation:
The event that happened last in the sequence provided is C. Northern troops marched triumphantly through Richmond, Virginia. The timeline of events during the Civil War starts with Confederate forces attacking Fort Sumter on April 12, 1861, which marked the beginning of the Civil War.
This is followed by various significant battles and strategic movements, including the Union Army laying siege to Petersburg, Virginia, which began in June 1864 and lasted until March 1865. The final major event is when Northern troops marched through Richmond, Virginia, which occurred in April 1865 after the city was captured by Union forces, symbolizing the impending end of the war.