Which of the following statements is/are correct?
I. In vertebrates, blood flows twice through the heart in each cycle.
II. Amphibians and many reptiles have three-chambered hearts.
III. Fishes have only two chambers to their hearts. 

All the statements are correct. I. The separation of the right side and the left side of the heart is useful to keep oxygenated and deoxygenated blood from mixing. Such separation is useful in beings that have high energy needs and constantly use energy to maintain their body temperature. Ex: Mammals. Also, in vertebrates, Blood goes through the heart twice during each cycle in other vertebrates. This is known as double circulation. II. In some animals like reptiles, the body temperature depends on the temperature in the environment. Such animals have three-chambered hearts, and tolerate some mixing of the oxygenated and de-oxygenated blood streams. III. Fishes have only two chambers to their hearts, and the blood is pumped to the gills, is oxygenated there, and passes directly to the rest of the body. Thus, blood goes only once through the heart in the fish during one cycle of passage through the body. Hence, option D is correct.