The correct answer is: D. no article.
Oxford is one of the oldest universities in Europe. It is not the oldest, but it is one of the oldest. Therefore, we do not need to use an article.
The article “a” is used before singular nouns that refer to a general thing or concept. For example, “I like to read books.” In this sentence, the word “books” refers to all books in general.
The article “an” is used before singular nouns that begin with a vowel sound. For example, “I saw an elephant.” In this sentence, the word “elephant” begins with a vowel sound, so we use the article “an.”
The article “the” is used before singular nouns that refer to a specific thing or concept. For example, “I like to read the book that you gave me.” In this sentence, the word “book” refers to a specific book that the speaker has been given.
In the sentence “Oxford is one of the oldest universities in Europe,” the word “universities” refers to a general concept. Therefore, we do not need to use an article.