Which of the following is a prepositional phrase?

Words that usually precede nouns or pronouns and express their relation with other parts of the sentence are known as prepositions. One type of preposition is a prepositional phrase, which is a combination of a preposition and a noun. "On time", "out of stock", "at once", and "for a change" are a few examples of prepositional phrases. Thus, "at once" is the correct answer to this question.