Final answer:
Baron von Steuben, a German aristocratic immigrant, was instrumental in retraining the American army during the winter at Valley Forge.
Step-by-step explanation:
The correct answer to the question is a. Baron von Steuben. Baron von Steuben, a German aristocratic immigrant, played a crucial role in retraining the American army during the winter at Valley Forge. He implemented a thorough training course for Washington's troops and transformed the Continental Army into a professional force capable of standing up to the British and Hessian soldiers. His drill manual, Regulations for the Order and Discipline of the Troops of the United States, influenced military practices in the United States for many years to come.