What is defined as a set of rules governing language structure?

The concept that is defined as a set of rules governing language structure is syntax. Syntax refers to the arrangement of words and phrases to create well-formed sentences in a language. It encompasses the rules that dictate how different parts of speech (like nouns, verbs, adjectives, etc.) can be combined, which is essential for clear and effective communication. Understanding syntax allows speakers and writers to construct sentences that are grammatically correct and convey the intended meaning.

In this context, while style pertains to the distinctive way in which a writer expresses their thoughts, and tone reflects the writer's attitude toward their subject or audience, neither directly relates to the structural rules of language. Figurative language involves expressions that deviate from the literal meaning to achieve a more impactful or imaginative effect, but it is not concerned with the foundational rules of sentence structure. Therefore, syntax is the most accurate choice among the options presented.