Question

Which part of speech is not appropriate in formal writing, but often used in daily conversation?

Answer 1

I think you're referring to slang?

Answer 2

Oh I think it's the use of contractions~ We use them daily, but what a lot of people don't know is that their use in a formal piece is not appropriate~

Answer 3

It's either contractions or interjections. Slang isn't a part of speech if it's referring to nouns/verbs/adjectives/etc