That is an interjection. The general rule is: Anything followed by a '!' is an interjection. A conjunction would be a word like "and" or "because." It joins stuff together. So, that's off the list. A preposition is an object's position or something with respect to time. So, that's off the list as well. An adverb describes an adjective and Yes, is not an adjective. So, the answer is Interjection.
