Answer:
not needed
Explanation:
Remember that a prefix is an affix that goes attached in the beggining of a word, phrase or base, it creates a derivative word.
In this case, un- means "no", it gives negative or opposite force in adjectives as well as their derivative adverbs and nouns.
As the word "necessary" means something that must be done, by using the un- prefix we can infer that it is not needed, it is not necessary.
