Study the following phrase. Which part of speech is the underlined word? 'but delayed until a quiz …'

The word 'until' in the phrase 'delayed until a quiz' functions as a preposition because it shows the relationship between 'delayed' and the noun phrase 'a quiz' (indicating time). When 'until' is followed by a noun/noun phrase, it acts as a preposition; when followed by a clause, it acts as a conjunction. Here it is followed by 'a quiz' (a noun phrase), so it is a preposition. Hence option (4) is correct.