Here's why:
* Common noun: A common noun refers to a general person, place, or thing (e.g., dog, city, chair). "Western states" is too specific to be a common noun.
* Proper noun: A proper noun refers to a specific person, place, or thing and is capitalized (e.g., John, Paris, Amazon River). While "Western states" refers to a specific group, it's not a proper noun.
* Collective noun: A collective noun refers to a group of people, animals, or things considered as a single unit (e.g., team, family, herd). "Western states" represents a collection of individual states, functioning as a single entity.
Therefore, "Western states" is a collective noun.