Answer:
Noun.
Step-by-step explanation:
First we need to remember that adjectives are parts of speech which stand beside a noun and modify it. ( for example: big city)
Adjective clause is a type of a dependent clause that consist of a relative pronoun (which connects clause with the word we are describing), subject and a verb - they all together serve as an adjective. So, just like adjectives, they also modifies nouns.
E.g. The cookies that my Grandma bakes are very tasty.