The correct answer is D. no article.
Paper was first made in China in the 2nd century BC. It was invented by Cai Lun, a Chinese court official. He made paper from a mixture of tree bark, hemp, rags, and other fibers. Papermaking spread to other parts of Asia and Europe in the following centuries.
The article “a” is used before singular nouns that refer to a specific person, place, thing, or idea. The article “an” is used before singular nouns that begin with a vowel sound. The article “the” is used before singular nouns that refer to a specific person, place, thing, or idea that has already been mentioned or that is unique.
In the question “When was paper first made?”, the word “paper” is a common noun that refers to a general type of material. Therefore, no article is needed.