Which of the following are structural words?

Structural words (also called function words or grammatical words) are words that primarily serve grammatical functions rather than carrying lexical content. They include auxiliaries (is, has, will), prepositions (in, on, at), conjunctions (and, but, or), articles, and pronouns. All three categories listed—auxiliaries, prepositions, and conjunctions—are structural/function words. Hence (4) All of the above is correct.