Final answer:
Dolphins have the greatest social complexity due to their larger brain-to-body size ratio and increased folding of the cortical sheet.
Step-by-step explanation:
According to the information provided, an increased number of folds in the cortical sheets of the brain is associated with increased social complexity in mammals. Among the given options, dolphins have the greatest social complexity because they have a larger brain-to-body size ratio and increased folding of the cortical sheet, which is correlated with complex behavior. Therefore, the correct answer is d. Dolphins.