Final answer:
Bivalves and gastropods (option c) are the organisms that exhibit open circulatory systems, whereas flatworms have no circulatory system, cephalopods have a closed system, and most annelids also have a closed system.
Step-by-step explanation:
Organisms with open circulatory systems include many arthropods and mollusks. Specifically, option (c) Bivalves and Gastropods exhibit open circulatory systems. In an open circulatory system, a fluid called hemolymph is pumped by the heart through vessels that empty into the body cavity. The hemolymph then returns to the heart through openings called ostia. Flatworms (option a) do not have a circulatory system, cephalopods (option b) have a closed circulatory system, and while most annelids (option d) have a closed circulatory system, some polychaetes and leeches may have an open system.