Which suffix would you mostly likely add to a word to form an adjective? -est, -ity, -ness
The suffix, which you would mostly add to a word to form an adjective, is est. Examples of adjectives include funniest, cheapest and finest. An adjective is a describing word.
I wouldn't agree with any of those choices. The words shown by richyrich were all adjectives already before adding the suffix, the suffix don't make them adjective.