Answer: The structure of the sentence provided is d. compound-complex.
Explanation: A compound-complex sentence is a sentence made up of a compound sentence and a complex sentence. While a compound sentence is formed with two independent clauses, a complex sentence is formed with an independent clause and a dependent clause. The sentence provided combines these two types of sentence because it is made up of two independent clauses ("Cheryl made the frosting" and "Hannah made the gravy") and two dependent clauses ("While Paul made a cake" and "While Demitri mashed the potatoes"), which have been introduced by the subordinating conjunction "while". Moreover, the coordinating conjunction "and" has also been used.