Answer: The structure of the sentence provided is C) compound-complex.
Explanation: A compound-complex sentence is a sentence that contains two independent clauses and one or more dependent clauses. While independent clauses can stand alone as sentences because they express full ideas, dependent clauses cannot. The only dependent clause included in the sentence provided is "Because he needed to improve his grades", which has been introduced by the subordinating conjunction "because". Moreover, the two independent clauses are "Joseph attended after-school tutoring" and "He worked harder on his homework", which have been joined by means of the coordinating conjunction "and".
