Final answer:
The circulatory plan of cephalopods is most similar to that of birds and mammals due to both having a closed circulatory system, which supports complex and active lifestyles.
Step-by-step explanation:
The circulatory plan of cephalopods is most similar to that of birds and mammals. Cephalopods, such as squids, have evolved a closed circulatory system, which is characteristic of more active and complex organisms. This system, where blood is contained within vessels and circulates efficiently to meet high metabolic demands, supports their active hunting lifestyle. On the other hand, other options like crustaceans and insects have an open circulatory system, which is less efficient than the closed system found in cephalopods, birds, and mammals. Plants, being vastly different in their physiology and lacking a circulatory system entirely, are irrelevant in this context.