Answer:
The suffix is -ness and it means something has a certain quality.
Explanation:
When we analyze the word "quietness" we find that a suffix was added to the adjective "quiet" and transformed it into a noun. The suffix that did that job is -ness, an English originated suffix that can be added to adjectives or participles. This suffix forms abstract nouns denoting quality or state. Thus, quietness is the state of being quiet. Other examples with the suffix -ness are: tiredness, darkness, goodness, restlessness etc.
