Final answer:
The dolphin is the organism with a fusiform shape, which is a streamlined body form that helps to decrease drag in water. This evolutionary adaptation allows dolphins to reach high swimming speeds.
Step-by-step explanation:
The organism that has a fusiform shape is the dolphin. A fusiform shape is a streamlined body form that is tapered at both ends. This adaptation is particularly useful for animals that live in water, as it helps to decrease the drag on the body as they move through water and allows them to swim at high speeds. Examples of animals with this body shape include certain sharks, which can swim at fifty kilometers an hour, and dolphins, which can reach speeds of 32 to 40 kilometers per hour. The fusiform shape is an evolutionary response to the forces of drag in the water, which has higher viscosity than air and thus exerts more resistance on moving objects.
Regarding the given multiple-choice question, dolphin is the correct answer, as dolphins are aquatic mammals with a fusiform body shape. Fish also have fusiform shapes but are not listed as options in the provided list. Birds, insects, and mammals (other than aquatic ones like dolphins and whales) typically don't have fusiform shapes because their adaptations are mainly for reducing the effects of gravity, not drag from water.