The correct answer is D. No article.
The word "middle" is a noun that refers to the central point or part of something. It is not preceded by an article because it is not a specific middle. There is no one specific middle of the night, as it can vary depending on the time zone and the time of year.
The other options are incorrect because they are all articles. An article is a word that is used to modify a noun. The articles "a" and "an" are used before singular nouns that begin with a consonant sound. The article "the" is used before singular nouns that are specific or unique. In the sentence "I woke up in the middle of the night," the noun "middle" is not specific or unique, so it does not need an article.
