Final answer:
Fish have the simplest closed circulatory system among the options provided, with their two-chambered heart allowing for separate circulation of blood within the body.
Step-by-step explanation:
The animal with the simplest closed circulatory system is the fish, which has a two-chambered heart consisting of one atrium and one ventricle. In closed circulatory systems, blood is kept separate from the interstitial fluid of the body and is transported within vessels, making this type of system distinct from the open circulatory system found in most arthropods and many mollusks. Although simple compared to the more complex circulatory systems of other vertebrates, the fish's system is efficient for its aquatic environment, adapting to their anatomy and evolutionary needs.