The correct answer is A. Starfish.
A chordate is an animal that has a notochord, a hollow nerve cord, and pharyngeal slits at some point in its life. The notochord is a flexible rod that runs along the back of the animal and provides support. The hollow nerve cord is a tube that runs along the back of the animal and contains the brain and spinal cord. The pharyngeal slits are openings in the pharynx that allow water to flow in and out of the animal.
Starfish are invertebrates, which means they do not have a backbone. They do not have a notochord, a hollow nerve cord, or pharyngeal slits. Therefore, they are not chordates.
Dolphins, sharks, and snakes are all vertebrates, which means they have a backbone. They all have a notochord, a hollow nerve cord, and pharyngeal slits at some point in their lives. Therefore, they are all chordates.
In conclusion, the correct answer is A. Starfish.