The word ending -ing is an example of a:

The word ending -ing is classified as a morpheme, which is the smallest meaningful unit of language. Morphemes can be either free, meaning they can stand alone as words (like "cat"), or bound, meaning they do not stand alone and must attach to a free morpheme to convey meaning, like the suffix -ing. This suffix adds information about the tense or aspect of a verb, indicating an ongoing action (e.g., "running," "singing").

In contrast, phonemes are the smallest units of sound in language and do not carry meaning on their own. A lexeme is a unit of meaning that may consist of one or more morphemes but is more focused on the vocabulary aspect than the structural component of language. Lastly, a syllable is a unit of pronunciation that typically includes a vowel sound and may or may not stand alone, but it does not convey meaning by itself and is not specifically tied to word formation as morphemes are. Therefore, the correct classification of -ing as a morpheme highlights its role in modifying and providing grammatical information within a word structure.