Martin Luther primarily opposed the challenges to the Church's authority and the belief that salvation was dependent on faith alone. Specifically, he contested the Church's practices and the selling of indulgences, which he believed undermined true faith. Therefore, in the context of the choices provided, you might be looking for the response that aligns with his theological disagreements, particularly about salvation and the Church's practices.
Among the choices listed:
- "challenges to the Church's authority" is the correct aspect he opposed because he challenged the Catholic Church's teachings and authority in general.
- He certainly supported the belief that salvation was dependent on faith, which makes the mention of that belief in opposition misleading.
Thus, selecting "challenges to the Church's authority" is the most appropriate response.