Final answer:
The three key 16th-century reformers who influenced most Protestant churches' doctrines and practices were Martin Luther, John Calvin, and Huldrych Zwingli.
Step-by-step explanation:
The Protestant Reformation was a religious reform movement that swept through Europe in the 1500s. It resulted in the creation of a branch of Christianity called Protestantism, a name used collectively to refer to the many religious groups that separated from the Roman Catholic Church due to differences in doctrine.
The three most important 16th-century reformers, whose ideas became the basis for the doctrine and practices of most Protestant churches, were Martin Luther, John Calvin, and Huldrych Zwingli. Therefore, the correct answer is A) Martin Luther, John Calvin, and Huldrych Zwingli. These reformers played crucial roles in shaping the religious landscape during the Reformation. Martin Luther's actions led to the Protestant Reformation, John Calvin founded Calvinism, and Huldrych Zwingli introduced radical changes in Zurich, challenging several core Catholic doctrines and practices.