The true answer is that we do not know (it’s difficult to measure intelligence in humans let alone dolphins) but dolphins seem to be very intelligent.
They can learn games really fast and they can generalize (which is true for other animals as well). Also, they can learn to understand language commands (which is true for other animals as well). Not only do the dolphins understand the meaning of individual words, they also understand the significance of word order in a sentence.
Find out more about how dolphins talk.