Final answer:
The animal group that is entirely aquatic is fishes. Amphibians, mammals, and reptiles all have terrestrial stages or members, while fishes, including both cartilaginous and bony species, are completely adapted to aquatic lives. Option d is the correct answer.
Step-by-step explanation:
Entirely Aquatic Animal Groups
When considering which of the following groups is entirely aquatic, we must examine their life cycles and habitats. Amphibians are not entirely aquatic; they have a dual life stage. They begin life in the water, breathing through gills, but then metamorphose and move onto land, breathing through lungs and their skin.
Mammals also do not fit this category as many are terrestrial, and although some, like whales and dolphins, did adapt to live in the ocean, they are not the entire group. Reptiles are primarily terrestrial; key adaptations like scaly skin and the development of amniotic eggs allow them to live on land, though some are aquatic.
The correct answer is fishes. These animals are entirely aquatic and include both cartilaginous fishes like sharks and rays, as well as bony fishes which are the largest group within Osteichthyes. All fish exhibit an aquatic lifestyle, though some may occasionally visit fresh water.