Final answer:
The correct answer is b. Corn syrup is denser, as indicated by the fact that the ping pong ball floats on top of it, which implies that the ping pong ball has a lower density than the corn syrup.
Step-by-step explanation:
When a ping pong ball floats on top of the corn syrup layer, it indicates a difference in density between the two substances. Density is an intensive property defined as the ratio of an object's mass to its volume (g/mL, g/cm³, etc.). Given that the ping pong ball is floating, we can infer that it is less dense than the corn syrup. Substances that are less dense than a fluid will float on its surface, much like oil floating on water or an iceberg in the sea. Therefore, the correct answer to the student's question is b. Corn syrup is denser.