Option 1 (sea cow) and option 3 (sea lion) are mammals because they possess characteristics typical of mammals, such as warm-blooded physiology, live birth, and the presence of mammary glands that produce milk for their young. In contrast, the sea horse, while part of the marine ecosystem, is a fish belonging to the family Syngnathidae and lacks the distinguishing features of mammals. This distinction is vital in biology and ecology, as it reflects the evolutionary adaptations of different classes of animals to their environments, which is an essential topic in the UPSC Prelims GS Paper I regarding biodiversity and the classification of living organisms.