The correct answer is Mammals. Explanation & Key Takeaways: • Mammals or Mammalia class of animal kingdom are found in a variety of habitats – polar ice caps, deserts, mountains, forests, grasslands and dark caves. • Some of them have adapted to fly or live in water. • The most unique mammalian characteristic is the presence of milk-producing glands (mammary glands) by which the young ones are nourished. • They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying. • The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. The heart is four-chambered. They are homoiothermous. Respiration is by lungs. Sexes are separate and fertilisation is internal. Related Concepts: • Insects • Entomology is the study of insects and their relationship to humans, the environment, and other organisms. • Entomology is crucial to our understanding of the human disease, agriculture, evolution, ecology, and biodiversity. • Crustaceans • Crustacea is a sub-phylum of the phylum Arthropoda. • It is a diverse group of arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, shrimps, mantis shrimp, etc. • The crustacean group can be treated as a subphylum under the clade Mandibulata. • Molluscs • Mollusca is the second-largest phylum of invertebrate animals after Arthropoda. • The members of the Mollusca family are known as molluscs. • Around 85,000 extant species of molluscs are recognized. • Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. • Numerous molluscs also live in freshwater and terrestrial habitats. • The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.