Correct answer is: B. ultimately
"Ultimately" fits the sentence to show the final outcome. "Ultimately" signifies the final or long-term outcome, which is the key point here. The word emphasizes that their cultural traditions persisted even after they interacted with other societies.
Why other options are incorrect:
Extremely: "Extremely" would suggest a high degree of something, but it doesn't convey the lasting nature of their cultural retention. It could imply a short-term or partial preservation.
Indispensably: This word means essential or necessary. While the Germanic tribes may have had a significant impact on the societies they settled in, their customs and ways of life were not strictly indispensable to their survival or continuation.
None of these: This is incorrect because the word "ultimately" clearly fits the context.