Answer: This sentence is written in a d) complex sentence structure.
Step-by-step explanation: A complex sentence structure consists in the presence of two clauses: one independent clause and one dependent clause. While an independent clause can stand alone as a sentence, a dependent clause cannot stand by itself because it does not express a complete idea. The sentence provided has a complex sentence structure because it includes an independent clause ("Rottweilers have also worked as guard dogs, draft dogs, and police dogs") and a dependent clause ("Although the breed started out as a Roman cattle dog"), which has been introduced by the subordinating conjunction "although".