What is an affix in linguistic terms?

An affix is defined as a bound morpheme that modifies or alters the meaning of a root word to which it is attached. Affixes can be prefixes, which are added at the beginning of a root word, or suffixes, which are added at the end. By changing the meaning or grammatical function of the base word, affixes play a crucial role in word formation and contribute to the richness of language. For instance, adding the prefix "un-" to the word "happy" forms "unhappy," effectively altering the meaning from positive to negative.

The other options do not accurately describe what an affix is. A morpheme that stands alone refers to free morphemes, which can function independently as words, and does not involve the concept of an affix. A rule for sentence structure pertains to syntax rather than morphology, the study of the structure of words. A minimal unit of sound describes phonemes, which are the smallest units of sound in a language and not related to the concept of affixes. Therefore, the definition of an affix as a bound morpheme that modifies meaning is the most precise and relevant.