Final answer:
The Reformation was sparked by Martin Luther's 95 Theses, which were posted on the door of the Castle Church in Wittenberg, Germany, in 1517. This led to the Protestant Reformation.
Step-by-step explanation:
The Reformation was sparked by the posting of Martin Luther's 95 Theses. On October 31, 1517, Martin Luther, a German monk and Professor of Theology, nailed his Ninety-Five Theses to the door of the Castle Church in Wittenberg, Germany. This act was intended to protest against practices such as the sale of indulgences and to call for reforms within the Catholic Church. However, rather than reforming the church, Luther's theses set off a chain reaction leading to the Protestant Reformation. This significant event in history ultimately resulted in the creation of various Protestant denominations and major changes within the Catholic Church itself.