Final answer:
The snake and starfish are not in the same class because the snake has bilateral symmetry while the starfish has radial symmetry, which is a principal characteristic that differentiates animal groupings.
Step-by-step explanation:
The snake and the starfish are not in the same class as other animals primarily due to their differing body symmetries. Snakes exhibit bilateral symmetry, meaning if you divide them down the middle, each side is a mirror image of the other. On the other hand, starfish display radial symmetry, wherein multiple planes can divide the animal into equal halves emanating out from a central axis.
In biology, body symmetry is a fundamental aspect of an organism's structure and is used to categorize animals. Sponges are asymmetrical, lacking any symmetry, while other groups like cnidarians have radial symmetry. More complex animals, including most vertebrates like snakes, possess bilateral symmetry which is associated with a more directed movement and a higher degree of specialization of body structures.
Therefore, the correct answer to the student's question is d) Radial vs. bilateral symmetry.