Which of the following is an example of a derivational suffix?

A derivational suffix is a morpheme added to a word to create a new word or change the grammatical category of that word. In this context, the suffix "-ness" is an example of a derivational suffix because it converts an adjective into a noun, as seen in the transformation of "happy" to "happiness."

The suffix "-ing" is primarily a grammatical suffix that marks the present participle form of verbs. Similarly, "-ed" typically denotes the past tense for verbs and is therefore not used to create a new word category. The suffix "-es" is primarily used to form the plural of nouns or the third person singular of verbs, thus also not being a derivational morpheme. In contrast, "-ness" distinctly modifies the base word's meaning and category, exemplifying the function of a derivational suffix effectively.