The correct answer is “Critical statements about the Church nailed to a church door”
At that time indulgences were a big business for the church. Rich people would pay the church huge amounts of money so they could receive indulgences and this way be forgiven and “go to heaven”. The Bible wasn’t in the center of the faith anymore and corruption was at its peak.
Martin Luther did not agree with that, because of that he wrote 95 theses and nailed it to Schlosskirch Wittenberg doors on October 31st of 1517. This made the Catholic church make a series of reforms starting with the Council of Trento.