Final answer:
Maria identified Bob's costume as an amphibian, which means Bob is dressed as a frog since it is the only amphibian listed in the options provided.
Step-by-step explanation:
If Maria said, "You're an amphibian!" while Bob was wearing a costume, it suggests that Bob is dressed as an animal that spends parts of its life both in water and on land. The correct animal from the options given that fits the description of an amphibian is a frog. Frogs typically have moist skin and can live both in aquatic and terrestrial habitats. Although toads are also amphibians, they generally have dry, bumpy skin and are not listed as an option. Therefore, the correct answer to which animal Bob is would be C. Frog.