Which suffix denotes 'state of' or 'quality of'?

The suffix that denotes 'state of' or 'quality of' is typically "ity" or "ty." These suffixes are used to form nouns that express a condition or quality associated with an adjective. For instance, the word "purity" reflects the state of being pure, while "activity" reflects the quality of being active. This morphological behavior makes "ity/ty" the correct choice for this question, as it signifies a specific state or quality rather than other concepts expressed by the other suffixes.

The suffixes "ment," "ness," and "er" serve different grammatical purposes; for example, "ment" often indicates an action or resulting state, "ness" denotes a quality (but is often used differently than "ity"), and "er" generally refers to someone who performs an action. Thus, "ity/ty" is uniquely suited to express states or qualities.