Which of the following vertebrates has the most simple structured heart

Fish have the most simple structured heart among the listed vertebrates. In fish, the heart is typically composed of two chambers: the atrium, which receives blood, and the ventricle, which pumps blood out to the gills and the rest of the body. This simple structure is sufficient for the needs of fish, which have a relatively low metabolic rate and a simple circulatory system. In comparison, reptiles, mammals, and amphibians have more complex hearts with additional chambers and more sophisticated circulatory systems to support their higher metabolic demands. For example, reptiles have a three-chambered heart with two atria and one ventricle, while mammals and birds have a four-chambered heart with two atria and two ventricles, which helps to separate oxygenated and deoxygenated blood.