A compound sentence is made up of two independent clauses, usually connected by a coordinating conjunction (such as "and," "but," "or," "nor," "for," "so," "yet").
The correct answer is:
A) AJ worked hard on his science fair project, yet he did not finish first.
This sentence contains two independent clauses: "AJ worked hard on his science fair project" and "he did not finish first." They are connected by the conjunction "yet."
(Note: There is a spelling error in the option, "finished" should be "finish.")