Final answer:
Martin Luther, the German monk, initiated the Protestant Reformation by challenging the Catholic Church's practices, particularly the sale of indulgences, leading to widespread changes across Europe.
Step-by-step explanation:
The German monk who began to question the teachings of the Catholic Church in 1517 was Martin Luther. This marked the beginning of the Protestant Reformation.
Luther, an Augustinian monk and a professor at the University of Wittenberg, was deeply troubled by the practices of the Catholic Church, particularly the sale of indulgences. His intellectual struggle with the church's teaching on salvation led to his writing of the Ninety-five Theses, which he allegedly posted on the door of Wittenberg's Castle Church as a call for debate and reform within the Church.
Luther's theses challenged the authority of the Pope and the practice of selling indulgences. Despite his initial intention to reform the Church, his ideas were perceived as a threat to the papal authority, leading to his excommunication. But with the advent of the printing press, Luther's criticism against the Church, including the declaration that faith alone was needed for salvation, spread rapidly throughout Europe, sparking widespread religious, cultural, and political changes.