Answer:
The sale of indulgences.
Step-by-step explanation:
Indulgence is a term that was used by the institution of the Roman catholic church to refer to total or partial remission of sins. The church would sell indulgences for all sins, each for with an appropriate amount of money.
The sale of indulgences was Luther's primary criticism as he believed that the remission of sins was only achieved through faith and honest repentance.