Final answer:
The institution that began to decline during the Protestant Reformation was the Catholic Church. Martin Luther's actions initiated this period of religious upheaval, leading to the formation of Protestant churches and diminishing the Catholic Church's pivotal role in Europe.
Step-by-step explanation:
One of the institutions that began to decline was the d. Catholic Church. Despite the enduring importance of religion, new religious ideas were emerging and bringing about significant change. Martin Luther's challenge to the Catholic Church marked the beginning of the Protestant Reformation, fundamentally altering the religious landscape of Europe. It led to the formation of Protestant denominations and dramatically reduced the Catholic Church's influence over European societies and politics.
The Protestant Reformation started with Luther's posting of the 95 Theses in 1517, criticizing the church's sale of indulgences and other corrupt practices. This act sparked a movement that saw many people in northern Europe break away from the authority of the Vatican and form their own Christian communities. The wave of religious change was unstoppable despite the Catholic Church's attempts, including the Council of Trent, to reform and retain its followers.