Concept:The heart structure becomes more complex as vertebrates evolve from fish to mammals.Explanation:Fish have a two-chambered heart (one atrium and one ventricle).Amphibians have a three-chambered heart.Reptiles also have a three-chambered heart, though with a partial septum.Mammals have a four-chambered heart, the most complex.Therefore, the fish has the simplest structured heart.Answer:B. fish