It specifies the rules which govern the arrangement of words into phrases, clauses and sentences.

Correct Option: Syntax

In linguistics, the study of language structure is divided into several distinct branches, each focusing on a different aspect of grammar and meaning:

1. Syntax: Refers to the set of rules, principles, and processes that govern the structure of sentences in a given language, specifically how words are combined to form phrases, clauses, and sentences.
2. Semantics: Focuses on the meaning of words, phrases, and sentences rather than their grammatical arrangement.
3. Discourse: Refers to units of language longer than a single sentence, dealing with how sentences combine in conversation or written text to form coherent communication.
4. Cohesion: Refers to the grammatical and lexical links within a text or sentence that hold it together and give it meaning.

Therefore, the term that specifically describes the rules governing word order and sentence structure is Syntax.