Which one of the following animals has the largest brain in proportion to its body size?

Dolphin is a cetacean mammal. Its brain is the largest in proportion to its body size. 

All cetaceans have the highest brain-to-body weight ratio, but dolphins have the highest among the rest. Bottlenose dolphins have larger brains than humans, with a brain-to-body-weight ratio that is higher than that of great apes.