What role did religion play in European conflicts?

Religion has played a very important role in European conflicts, especially in the Middle Ages and the Early Modern Age.

Explanation:

For example, in the Middle Ages, many conflicts arose between Eastern Europe, which was still pagan, and Western Europe, which had fully christianized.

The Crusades were also a series of religious wars between European Christians, and Muslim Middle-easterners, for the control of the Holy Land.

In the Early Modern Era, the Protestant Reformation led to a Catholic counter-reformation movement, that ignited the 30 years war, one of the bloodiest wars in European history until the two world wars, and which was fought mainly in Germany, between Protestants and Catholics.

d is the best answer for this question.