An antonym for flounder would be graceful.
Answer:
Interjection.
Explanation:
An interjection is a grammatical part of speech that demonstrates the feelings and emotions of a person. And interjections are mostly followed by an exclamation mark, which makes it easy to identify.
In the given sentence, the word "Hey" is an interjection even though there is no exclamation mark after it. It is used to get the attention of the others before saying what one wants to say.
Thus, the correct answer is an interjection.
