Answer: The right answer is the B. In America, Edgar Allan Poe was long regarded as a minor writer—an author of melodramatic poems and thrilling tales of mystery and suspense.
Explanation: Just to elaborate a little on the answer, it can be added that the marks of punctuation, specifically the comma and the so-called "em dash," in this sentence have been correctly used, since, on the one hand, the em-dash has been used to set apart an additional comment or an aside from the main clause, which states that "Edgar Allan Poe was long regarded as a minor writer," with the goal of emphasizing that comment and elaborating further on the idea stated in the main clause, and, on the other hand, the comma has been used following an introductory phrase, which announces to the reader that a subject and a verb come later.