________ is the most intelligent mammal on earth

The correct answer is Option 3, i.e Dolphins.

Key Points

• Dolphins are the most intelligent mammals on earth. 
• Studies show that they are capable of complex problem solving and have the ability to learn as an individual.
• They can also pass the knowledge on to others.
• Dolphins have highly-developed brains. Research shows that dolphins mirror self-recognition, cultural learning, comprehension of symbol-based communication systems, and an understanding of abstract concepts — comparable to chimpanzees and other great apes.
• Dolphins use echolocation for hunting. They emit "clicks" which work on the principle of echo.
• Dolphins analyze the echo from the reflected objects as quickly as they keep on swimming.
• Their echolocation can even help them know the kind and size of the objects.