Final answer:
Calvinism was recognized as legitimate within the Holy Roman Empire for the first time following the Thirty Years' War, as per the Peace of Westphalia in 1648.
Step-by-step explanation:
At the end of the Thirty Years' War, the religion that was recognized as legitimate within the Holy Roman Empire for the first time was Calvinism (B). Prior to the Thirty Years' War, the 1555 Peace of Augsburg had only recognized Lutheranism alongside Catholicism, allowing rulers to choose between the two for their territories. However, after the prolonged and devastating conflict of the Thirty Years' War, the Peace of Westphalia in 1648 expanded this recognition, establishing not only the independence of the states within the Empire but also acknowledging Calvinism as a legitimate option for rulers in addition to Catholicism and Lutheranism. This marked a significant point in European history where religious plurality was officially accepted within the Empire.