El bus llega las ocho horas.
por
para
a
en

The words from the list are prepositions. A preposition is a word or even a group of words, that we use when showing the way which other words are connected indicating a relationship between two words or phrases, nouns, nouns and verbs, and between different parts of a sentence. So the preposition a can be used to express time translating into at or is. Finally:

El bus llega a las ocho horas

do you know the title of the listening so i can hear and answer this?

if your asking for what that says, it is

which of these words is an

but i don't know what sentence your asking for the adjective from.