The correct answer is: the practice of selling indulgences.
The 95 Theses were written in 1517 by Martin Luther, professor of moral theology at the University of Wittenberg, Germany.
Luther objected to the corrupt practice of selling indulgences because he believed that salvation could only be reached through faith and by divine grace. His 95 Theses would later become the foundation of the Protestant Reformation which profoundly changed Europe.