Answer:
C: Machine Gun.
Step-by-step explanation:
The invention of the machine gun during the American Civil War greatly aided in holding the line against enemy attacks. The Gatling gun, invented in 1861, was an early example of a practical machine gun that could fire multiple rounds per minute. Its high rate of fire allowed a small number of soldiers to deliver a significant volume of fire, making it difficult for the enemy to advance. This advantage in firepower enabled defenders to repel assaults and hold their positions more effectively.