The Hudson River was one the chief trading route for the western frontier.
The phrase is for the western frontier.
Its an adjective phrase because it describes was once the chief trading route right?
It's describing "route" -- yes, correct.
its an adjective phrase right?
right
Actually, the phrase "for the western frontier" in the sentence "The Hudson River was one the chief trading route for the western frontier" is a prepositional phrase. It begins with the preposition "for" and provides more information about the purpose or destination of the chief trading route. Prepositional phrases typically start with a preposition and end with a noun, pronoun, or gerund.
To identify the type of phrase in a sentence, you need to analyze the structure and function of the words within the phrase. In this case, the phrase "for the western frontier" is a prepositional phrase because it starts with a preposition and is providing more information about the trading route, rather than directly describing it.