Preposition is a word (or several of them) that usually comes before a noun or a pronoun, its role is to show us the connection between a noun/pronoun and other parts of a sentence. In the given example, preposition "underneath" in the adverbial phrase "underneath the mattress" helps us to define a place of an object (secret envelope).
the answer to your question is D) The three categories of doughnuts are glazed, filled, and cake; they are named by their characteristics and ingredients.
