What makes the following sentence a compound sentence? Selected for a national baking show, Mariana was ecstatic, and she planned to create the best cake of her life to win the grand prize.

What makes the sentence a compound sentence is that it has at least two independent clauses and no dependent clause.

Explanation:

Where two or more independent clauses are contained in a sentence, it is known as a compound sentence. It also includes the use of a comma, semicolon and coordinating conjunctions like "and", "for", but, "nor', "yet", "so".

other examples of a compound sentence are, "I called Mary but she didn't answer her phone", "I winked at her and she smiled back at me", 'the light went off so I turned on the lamp".