A, B, D - Incorrect: Consider does not take a preposition, so ...considered as/to be or consider... as is the wrong idiom here.
B - Incorrect: It is very unlikely that the plummy flavor was noteworthy for being virtually unheard of outside Navarra. Placing this last phrase as a modifier after flavor changes the meaning of the sentence.
D - Incorrect: The idea that locals "consider the drink fit for a gourmet" is lost; instead, this sentence incorrectly states that locals "consider it virtually unheard of..."
E - Incorrect: It is unclear whether the introductory phrase modifies most locals or Tempranillo, though of course the first option makes little sense in the sentence.
C - Correct: considered a gourmet drink is the best use of the idiom in this passage.
C is the correct answer.