The correct answer is (B).
Getting to the Answer:The underlined portion, "more prettier," compares Granada to other cities in Spain. However, since more than two subjects are being compared, the superlative "prettiest" should be used. Additionally, the comparative form in English would use either "more" or the comparative word with the -er ending, but not both.
(A) Incorrect: the superlative adjective "prettiest" is required to compare more than two subjects, and "more prettier" is not correct English usage
(B) Correct: the superlative "prettiest" is required to compare Granada to the other cities in Spain
(C) Incorrect: the comma after "prettier" would incorrectly separate the adjective from the noun it describes ("cities"), and the superlative form "prettiest" is required
(D) Incorrect: "most prettiest" is not correct English usage; the superlative form in English uses either "most" or the superlative word with the -est ending, but not both