Final answer:
In the provided sentence, 'like' acts as a preposition that introduces a comparative phrase.
Step-by-step explanation:
The student asked what part of speech the underlined word 'like' is in the sentence 'When she came up, she broke the surface smoothly, gracefully, like a swan.' In this sentence, the word 'like' functions as a preposition because it introduces the phrase 'like a swan' which serves to compare 'how she came up' to the gracefulness of a swan. Prepositions are words that show the relationship between a noun or a pronoun and other words in a sentence, often indicating time, place, or manner.