The word 'in' would be a preposition in the example sentence you gave.
"Interested in" is a prepositional phrase typically used to indicate a preference or desire for something. It functions as an adjectival phrase modifying a noun or pronoun in a sentence.
regular speech
it depends what word it is it could be any part of speech depending on the sentence
The action in a sentence is the part of speech known as a verb.
The part of speech of "correct" in this sentence is an adjective.
The part of speech for "gliding" is a gerund, which functions as a noun in a sentence.
All subjects are part of speech for they are all nouns.
The part of speech that answers "what" or "whom" in a sentence is a pronoun. Pronouns are words like "he," "she," "it," "they," "who," and "what" that replace nouns in a sentence.
a sentence is a part of speech
Why is the beginning of a interrogative sentence.
you