The correct answer is A. The sky, a blue blanket, stretched beyond the Pacific Ocean
Step-by-step explanation
In the sentence, we see the use of the punctuation mark called "comma" which is used to separate words, elements of a sentence, or sentences. In the case of the question of the sentence presented, the comma should be used to separate the section "a blue blanket" because this element works as an extra detail that redefines the noun of the sentence "The Sky". Therefore, the correct answer is A. The sky, a blue blanket, stretched beyond the Pacific Ocean.