Marine mammals are species that rely on the ocean for their existence and include groups such as cetaceans (whales, dolphins, porpoises), pinnipeds (seals, sea lions, walruses), sirenians (manatees, dugongs), and others. Let's evaluate each option:
A. Dolphin
Marine Mammal: Yes, dolphins are cetaceans and are marine mammals.
B. Walrus
Marine Mammal: Yes, walruses are pinnipeds and are marine mammals.
C. Sea Horse
Marine Mammal: No, sea horses are fish, not mammals.
D. Porpoise
Marine Mammal: Yes, porpoises are cetaceans and are marine mammals.
E. Dugong
Marine Mammal: Yes, dugongs are sirenians and are marine mammals.
F. Shark
Marine Mammal: No, sharks are fish, not mammals.
G. Seal
Marine Mammal: Yes, seals are pinnipeds and are marine mammals.
Count of Marine Mammals: Five (Dolphin, Walrus, Porpoise, Dugong, Seal)
Answer: (4) Only five
Sources
https://www.noaa.gov/education/resource-collections/marine-life/marine-mammals
https://marinemammalscience.org/science-and-publications/list-marine-mammal-species-subspecies/
https://www.centreofexcellence.com/species-of-marine-mammals/
