Final answer:
Marquis de Lafayette allied with the Continental Forces after the winter at Valley Forge, playing a significant role in the American Revolution and contributing to the American victory at the Battle of Saratoga. The correct option is d).
Step-by-step explanation:
The individual who decided to ally with the Continental Forces after the winter at Valley Forge was Marquis de Lafayette. He was a French aristocrat and military officer who played a key role in the American Revolution, offering his assistance to General George Washington and becoming a crucial figure at Valley Forge and in subsequent battles.
Lafayette's contribution was particularly significant at the Battle of Saratoga, which turned the tide of the war in favor of the Americans. This battle, along with Lafayette's actions, convinced France to enter the war on the side of the American colonies, leading to a formal alliance that proved critical to the ultimate success of the American cause.
General George Washington himself was already aligned with the Continental Forces, being their commander in chief and leading them through several battles, including the famous crossing of the Delaware River to surprise the Hessians at Trenton, New Jersey. Benedict Arnold also played an earlier role on the American side before his notorious defection to the British.