Correct answer:
a
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