This question is testing the recognition of adverbial inserts in sentence structure. The correct answer is 'A. 'Fortunately, they had brought an umbrella.' because 'fortunately' is inserted into the sentence to modify the entire clause, expressing a sentiment about the situation. Option B is incorrect because 'indeed' is used at the end of the clause to emphasize the previous statement. Option C is incorrect because 'indeed' is not inserted within the clause but is used as an emphasis at the beginning. Option D is incorrect because 'however' is used at the end of the clause to indicate contrast, not as an insert.