Final answer:
The Holy Roman Emperor who sought to suppress the Lutheran Reformation was Charles V. His efforts were undermined by the Peace of Augsburg in 1555, which allowed rulers to choose Lutheranism as their official religion.
Step-by-step explanation:
During the Lutheran Reformation, the Holy Roman Emperor who unsuccessfully sought to suppress the new faith was Charles V. Charles V was a staunch Catholic who opposed the spread of Protestantism brought about by Martin Luther's teachings. Despite his efforts to maintain Catholic dominance, the Peace of Augsburg in 1555 marked a turning point by allowing rulers to establish Lutheranism as the official religion in their territories if they so desired. This was a significant setback for Charles V's agenda and indicated the increasing power and influence of the Lutheran movement within the Holy Roman Empire.