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

The correct option is Syntax.

Explanation:

Syntax refers to the set of rules, principles, and processes that govern the structure of sentences in a given language, specifically determining how words are arranged to form coherent phrases, clauses, and sentences.

Let's look at the other options to understand why they are incorrect:

• Discourse: Refers to continuous stretch of spoken or written language larger than a sentence, focusing on context and communication beyond individual sentences.
• Semantics: Refers to the branch of linguistics concerned with the study of meaning in language.
• Cohesion: Refers to the grammatical and lexical links that hold a text together and give it meaning, creating unity across sentences.