What act of the roman catholic clergy did martin luther most strongly criticize in his ninety five theses

Martin Luther is popular for criticizing the Indulgence-selling of the Roman Catholic church. It is the act of selling "absolution" of sin to the patrons, this type of activity soon became corrupt. Luther abhorred by this practice wrote the 95 theses, which fundamentally has two main arguments. First was sinner should ask for repentance and salvation from God himself. And the latter was the argument that instead of faith alone, deed should be practiced to achieve salvation. He posted this 95 theses in the doors of Wittenburg Castle Church in October 31, 1517.