Which of the following best describes a phonological rule?

A phonological rule is best described as a principle governing sound change in speech. This refers to the systematic patterns that dictate how phonemes (the smallest units of sound) are organized and altered in speech across different contexts within a language. Phonological rules help explain why certain sounds can change in pronunciation depending on their surrounding sounds or position within a word.

For instance, in English, the plural morpheme can be pronounced as /s/, /z/, or /ɪz/ depending on the preceding sounds. This illustrates how phonological rules account for variations in speech patterns. Understanding these rules is crucial for analyzing how sounds function and vary in different linguistic environments.

In contrast, the other concepts listed do not pertain to phonological rules. Criteria for word formation relates to morphology, while guidelines for sentence structure are associated with syntax. An index of phonetic symbols would be more focused on the representation of sounds rather than the rules that govern their interactions.