Final answer:
The most powerful country in Europe after the Thirty Years' War ended in 1648 was France, which had sided with Protestant states and emerged as the dominant force in western Europe.
Step-by-step explanation:
When the Thirty Years' War ended in 1648, it became clear that France was the most powerful country in Europe, making 'A) France' the correct answer to the quiz question. The Peace of Westphalia significantly boosted France's status, as it emerged victorious and became the dominant force in western Europe. This outcome came after a series of conflicts, notably the participation of France on the side of Protestant states against the Catholic Habsburgs of the Holy Roman Empire.
When the Thirty Years' War ended in 1648, it made France the most powerful country in Europe. France, under the rule of King Louis XIV, emerged as a dominant force due to its military successes and territorial gains during the war.