Final answer:
The region of Europe mostly occupied by Catholics prior to the Protestant Reformation included western and central parts, with key Catholic nations such as Spain, France, and Italy, as well as parts of the Holy Roman Empire.
Step-by-step explanation:
The region of Europe that was occupied mostly by Catholics includes the western and central parts of the continent. During the thirteenth century and prior to the events of the Protestant Reformation in the 1500s, Christianity, in the form of Roman Catholicism, provided a unifying structure throughout these regions. The Vatican in Rome exercised considerable influence, including levying taxes and controlling learning. Major Catholic regions included Spain, which was immersed in wealth from the New World and stood as a bastion of the Catholic faith. The Protestant Reformation, initiated by Martin Luther in 1517, led to a split among European Christians, yet Roman Catholicism remained dominant in the western and central parts of Europe. This area would broadly include countries like Spain, France, Italy, and parts of the Holy Roman Empire, which encompassed regions of modern-day Germany, Austria, and others.