Which of the following parts of speech is used to join words or groups of words?

The correct choice is conjunction, as conjunctions serve the specific function of connecting words, phrases, or clauses within a sentence. This means they facilitate the relationship between different elements, allowing for the construction of more complex and meaningful sentences. For instance, common conjunctions include "and," "but," and "or," which can link items in a list or create contrast and alternatives between ideas.

Conjunctions are essential for coherence in writing, helping to maintain the flow of thought and making connections clear to the reader. In contrast, the other parts of speech mentioned—adverbs, prepositions, and interjections—serve different roles. Adverbs modify verbs, adjectives, or other adverbs; prepositions indicate relationships in time or space; and interjections express emotions or abrupt sentiments. Therefore, conjunctions are uniquely identified by their joining function, setting them apart as the correct answer.