Is "during" a pronoun, verb, adverb, or preposition?
It is a preposition.
The word "during" is a preposition, because it is never interpreted alone, but its meaning is always linked with a noun next to it. For example let's analyze a sentence:
During the match the referee showed 3 yellow cards.
If we want to answer a question "when?" we don't use the word during alone, but we add the word next to it "during the match"