The correct answer is: B. dolphins.
Dolphins are highly intelligent mammals that are closely related to whales. They have large brains, complex social structures, and the ability to learn and use tools. Dolphins are also known for their playful and friendly behavior.
Whales are also highly intelligent mammals, but they are not as closely related to dolphins as dolphins are to each other. Whales have large brains and complex social structures, but they do not have the same level of tool use as dolphins.
Elephants are the largest land mammals on Earth, and they are also highly intelligent. Elephants have large brains and complex social structures. They are also known for their long memories and their ability to use tools.
Kangaroos are marsupial mammals that are native to Australia. They are not as closely related to dolphins, whales, or elephants as these animals are to each other. Kangaroos have relatively small brains and simple social structures. They are not known for their tool use or their intelligence.
In conclusion, the most highly intelligent mammals are dolphins. They have large brains, complex social structures, and the ability to learn and use tools.