Which of the following is an example of an inflectional suffix?

The correct answer is the suffix -ing. This suffix is classified as an inflectional suffix because it modifies a verb to indicate ongoing action or the process of that action, as in "running" or "swimming." Inflectional suffixes, in general, serve to alter the grammatical function of a word without changing its core meaning, such as tense, number, or case.

The other suffixes listed are not considered inflectional. The suffix -ed is also an inflectional suffix as it is used to denote past tense for verbs; however, since it is not the chosen answer, it does not apply here. The suffix -ly is a derivational suffix that transforms adjectives into adverbs, while -ful is also a derivational suffix that modifies nouns to form adjectives signifying "full of." These derivational suffixes change the part of speech and meaning, rather than simply adjusting tense or number.