C. Jack and Jill went up the hill. This is because a simple sentence is made up of an independent clause. A compound is made up of both an independent and dependent clause A dependent clause is a phrase that cannot stand on its own. For example, "When we went to Brusters". This kind of clause needs to be paired with an independent clause to form a full thought/sentence. Independent clauses are phrases that can stand on their own as a simple sentence. For example, "I ordered strawberry ice cream" In your question's case, there is one independent clause, which is "Jack and Jill went up the hill". Compound sentences combine 1 independent and 1 dependent clause to form a sentence. If we combine my two examples by the use of a comma (but you can also use a coordination conjunction), we can form a compound sentence. "When we went to Brusters, I ordered strawberry ice cream." Complex sentences must include the elements of a compound sentence, so that is not correct. Since you only have one independent clause and nothing else, "Jack and Jill went up the hill" is an example of a simple sentence.
