The correct answer is: D. no article
Asia is the largest continent in the world, covering 44.579 million square kilometers (17.222 million square miles). It is located primarily in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa.
The article “a” is used before singular nouns that refer to a general or unspecified thing. For example, “I saw a dog.” The article “an” is used before singular nouns that begin with a vowel sound. For example, “I saw an apple.” The article “the” is used before singular nouns that refer to a specific or particular thing. For example, “I saw the dog that bit me.”
In the sentence “Asia is the largest continent in the world,” the word “Asia” is a proper noun, which is a noun that refers to a specific person, place, or thing. Proper nouns are always capitalized. In this case, the word “Asia” refers to the continent of Asia. Therefore, the article “a,” “an,” or “the” is not needed.