Answer:
The proper nouns in:
The last group of raiders to terrorize western Europe were the Vikings fierce Scandinavian warriors who almost overran England before King Alfred negotiated peace.
Vikings
Step-by-step explanation:
A proper noun is a word or concept used to identify or group a different amount of individuals or objects that belong to the same upper category. In our case, the proper noun in this text is the Vikings. Because they belonged o many different countries or civilizations, while they were all raiders also. So the debate would be between those two words, however not all raiders were Vikings. That is why The Vikings are the correct answer.