Final answer:
The Protestant Reformation was primarily caused by corruption in the Catholic Church and a call for spiritual transformation, leading Martin Luther to post his Ninety-Five Theses.
Step-by-step explanation:
A major reason for the Protestant Reformation was the corruption within the Catholic Church, including the sale of indulgences, and the opulent and immoral lifestyles of some of its clergy. This disillusionment, combined with a desire for religious transformation and scriptural authority, propelled figures like Martin Luther to challenge the Church's practices. Luther, a German monk, sparked the Reformation by posting his Ninety-Five Theses in 1517, which criticized Church practices and initiated debates that led to a split in Western Christianity and the formation of various Protestant denominations.