What act of the Roman Catholic clergy did Martin Luther most strongly criticize in his Ninety-Five Theses?

In his Ninety-Five Theses, Martin Luther most strongly criticized the sale of indulgences by the Roman Catholic clergy. He argued that these practices undermined true repentance and the core message of salvation through faith alone, suggesting that forgiveness cannot be bought. Luther believed that the clergy's actions exploited the faithful and distorted Christian doctrine, prompting a significant call for reform within the Church.