What practice did Luther protest concerning indulgences?

The correct answer highlights Martin Luther's primary concern with the sale of indulgences, which were essentially pardons sold by the clergy that promised to release individuals from penalties due to their sins. Luther believed that this practice encouraged corruption within the Church and distorted the true meaning of repentance and forgiveness. He argued that salvation could not be bought and that true faith and repentance were the only means to achieve it. This protest against the commercialization of salvation was central to his 95 Theses, which sparked the Reformation and initiated questioning of Church practices.

The other practices mentioned, while related to the broader context of Church corruption, do not directly address the specific issue Luther focused on regarding indulgences. Selling relics and charging fees for services were also grievances against the Church's financial practices, but they were not the central issue that ignited Luther's opposition.