Is South Dakota a common or plural noun?

The noun 'South Dakota' is a proper noun, the name of a US state; the name of a specific place.

A proper noun is the name of title of a specific person, place, or thing.

A common noun is a general Word for a person, place, or thing.

Examples of common nouns for the proper noun 'South Dakota' are state or place.

The noun 'South Dakota' is singular, there is only one South Dakota.

A plural noun is a Word for two or more people, places, or things.