The answer is ROOT.
Step-by-step explanation:
The word "unhappy" is formed by the prefix "un" (meaning "no") and the root "happy".
Morphology is the branch of linguistics that studies the structure within words. In this morphological process, the suffix "ness" can also be attached to the word "unhappy" to form "unhappiness". As we can see, each part of the word encodes meaning.