A prefix is an example of what linguistic unit?

A prefix is indeed an example of a morpheme, which is the smallest meaningful unit of language. Morphemes can be standalone words or parts of words, such as prefixes, suffixes, and roots. In the case of prefixes, they are attached to the beginning of a base word to modify its meaning, such as "un-" in "undo" or "pre-" in "preview." This modification is a key function of morphemes, making them essential components of word formation and helping to convey additional meaning.

The other options refer to different linguistic concepts. A word is a complete unit of meaning that may consist of one or more morphemes, while a syllable refers to a unit of pronunciation that typically contains a vowel sound and may or may not have consonants. A lexeme represents a basic unit of meaning before any inflections or variations are applied, encompassing the full range of a word's forms, but it does not specifically denote the types of modifications that prefixes signify.