The contribution of the soldiers at Valley Forge to the American Revolution primarily lies in their ability to endure a harsh winter and emerge as a more cohesive and skilled fighting force. Although they did not win a battle at Valley Forge, the experience there, including the training received under leaders like Baron von Steuben, significantly improved their discipline and tactics.
However, from the provided options, none accurately encapsulate the main contribution of the soldiers at Valley Forge. They did not win a major battle there, nor did they play a direct role in convincing the French to ally with the Americans, nor were they primarily responsible for defeating the British or spying on them.
The significance of Valley Forge is more about the resilience, training, and unity it fostered among the Continental Army, which ultimately contributed to their successes in later battles.