A. They challenged the authority of the Church.
The posting of Martin Luther's 95 Theses was a radical step because they challenged the authority of the Church. In the 16th century, the Church held a great deal of power and influence, and to openly challenge its authority was seen as a bold and potentially dangerous move. The Theses were a list of propositions for debate, and in them, Luther questioned many of the Church's practices and beliefs, including the use of indulgences and the authority of the Pope. By publicly airing these grievances, Luther was taking a stand against the Church and its teachings, which was a bold and radical act at the time.