Answer:
D. The boy worked hard on his story, for he wanted to win the contest.
Explanation:
A compound sentence is any sentence composed of two independent clauses, separated by a comma and a conjunction (for, and, nor, but, or, yet, so) or a semicolon. The two independent clauses in answer D are "The boy worked hard on his story" (subject being boy, verb being worked), and "he wanted to win the contest" (subject being he, verb being wanted).
Answers A and B are incorrect because they do not contain two independent clauses. C also does not contain two independent clauses, but instead a prepositional phrase, which could be confused with an independent clause. These are not the same however, and the comma separating this phrase and clause is not followed by a conjunction word, therefore it is not a compound sentence.
I hope this helps! Happy learning, friend :)
