Which of the nouns functions as an adjective in sentence Carlos wanted a new cage for his petferret?

The noun that functions as an adjective in the sentence is "pet".

Here's why:

* "Pet" modifies the noun "ferret", describing the specific type of ferret Carlos has.

* "Cage" is the object of the verb "wanted".

* "New" is an adjective describing the type of cage Carlos wants.

Therefore, "pet" functions as an adjective because it modifies another noun ("ferret") to provide more information about it.