interjection is correct because Fie! means an exclamation used to express blame, disapproval, or disgust.

This idiom or phrase is useful because it helps students understand the intended meaning beyond the individual words.

Examples: Fie! You should not speak so rudely. Fie! That was a shameful act.

Fie! stands alone as an exclamation, so it is an interjection, not a conjunction or prepositional phrase.