What is defined as the system of rules for how sounds are used in language?

Phonology refers to the system of rules governing how sounds are used in language. It encompasses the study of the sound systems of languages, including how sounds are organized and how they function in context. Phonology examines aspects such as phonemes, which are the distinct units of sound that can differentiate meaning, as well as the rules that dictate how these sounds interact with one another within a language.

Morphological rules pertain to the structure of words and how they are formed from morphemes, which are the smallest units of meaning. Orthography deals with the conventional spelling system of a language, focusing on the representation of sounds in written form. Syntax is about the arrangement of words and phrases to create well-formed sentences. Each of these concepts is vital to understanding language, but phonology specifically addresses the aspect of sound, making it the correct answer for the question.