The correct answer is:
b) The victory at Saratoga showed them that the United States could defeat Great Britain.
The Battle of Saratoga in 1777 was a turning point in the American Revolutionary War, as it demonstrated that the American forces could win significant battles against the British. This victory convinced France, and subsequently Spain, that the United States had a viable chance of winning the war, leading to their decision to support the American cause in order to weaken Great Britain.