identify the part of speech for underlined phrase was similar to the One he had seen in Yellowstone National Park

I managed to find the exercise online - you are supposed to determine what type of a noun Yellowstone National Park is.

The correct answer is a proper noun. Proper nouns are used to denote names; names of people, animals, geographical locations, rivers, parks, etc. So given that this is the name of a park, it is also a proper noun.