The correct answer is D. no article.
The word "mystery" is a non-count noun, which means it cannot be used with a definite article (the) or an indefinite article (a or an). Non-count nouns refer to things that cannot be counted individually, such as water, air, or love.
In the sentence "Billu enjoys reading mystery novels," the word "mystery" is used to refer to the genre of novels that Billu enjoys reading. Genres are non-count nouns, so they cannot be used with articles.
If we wanted to say that Billu enjoys reading a specific mystery novel, we could say "Billu enjoys reading The Hound of the Baskervilles." In this case, the word "The" is a definite article, which is used to refer to a specific person, place, thing, or idea.
We could also say that Billu enjoys reading mystery novels by Agatha Christie. In this case, the word "by" is used to indicate that Billu enjoys reading all of the mystery novels that Agatha Christie has written.
I hope this explanation is helpful!
