Final answer:
The key battle that shifted the war in favor of the Americans was the Battle of Saratoga, marking a significant turning point which also led to French support for the American Revolution.
Step-by-step explanation:
The battle that turned the tide of war in favor of the Americans during the American Revolutionary War is A. the Battle of Saratoga.
This crucial encounter, occurring in 1777, is widely considered a turning point in the war. The American victory at Saratoga not only boosted morale among the colonists but also played a pivotal role in garnering French support for the American cause. The assistance from France proved essential in sustaining and ultimately securing American independence.