Which of the following best defines a conjunction?

A conjunction is a key component of grammar, specifically designed to connect clauses, sentences, or phrases. The primary function of a conjunction is to provide linkage, allowing for a smooth flow of ideas and concepts within written or spoken language. This enhances coherence and clarity, making it easier for the reader or listener to understand the relationships between different parts of a sentence or multiple sentences.

In contrast, options that describe other parts of speech, such as adjectives, nouns, or possessive forms, do not fulfill the connecting role layed out for conjunctions. For example, adjectives describe nouns, nouns can serve various functions like being subjects or objects, and possessive forms indicate ownership. However, none of these functions involve the connective aspects that define conjunctions. Thus, the choice that identifies a conjunction as a word that connects clauses or sentences is indeed the best definition.