Final answer:
The Crusades demonstrated the power of the church in Europe by the direct involvement of church leaders in military campaigns, church financing of the Crusades, and the religious motivation provided to the Crusaders. The decline of crusading in the later Middle Ages was largely due to the loss of influence by the popes, among other geopolitical shifts.
Step-by-step explanation:
The Crusades are an example of how the power of the church was demonstrated in Europe through various means. Church leaders not only called for the Crusades, as seen with Pope Urban II, but also led military campaigns, signifying direct involvement and control over such endeavors. Significant funding and resources from the church also bolstered these campaigns, allowing the Crusades to be more than just ideologically-driven missions, but well-supported military ventures. Moreover, Crusaders were often motivated by religious objectives; to serve on behalf of their faith was considered a noble and honorable deed. Among the multiple-choice options provided in your question, options (a), (b), and (c) reflect the power of the church during the Crusades, while option (d) is incorrect as church influence did not diminish but grew during the era of the Crusades.
As for the reasons relating to the decline in crusading in the later Middle Ages, it was the decline of power and influence of the popes (option a) that contributed significantly. The Crusades had been launched by influential popes who were seen as reformers, but over time, their focus shifted towards their own power and prestige, which in turn, diminished the crusading ideal. Other historical facets, such as the emergence of strong Islamic kingdoms and the shift of European monarchs' focus towards nation-state building, also contributed to the decline of the Crusades.