Which group of animals is primarily classified as ectotherms?

Ectotherms, often referred to as "cold-blooded" animals, are those that rely on external environmental conditions to regulate their body temperature. The group that includes reptiles and amphibians predominantly exhibits this characteristic. Reptiles, such as snakes and lizards, and amphibians, such as frogs and salamanders, depend on the ambient temperature for their thermal regulation, affecting their metabolic rates and activity levels.

In contrast, birds and mammals are classified as endotherms, which means they can regulate their body temperature internally through metabolic processes. This allows them to maintain a stable body temperature regardless of the external environment. Fish, while they can vary in thermoregulatory strategies, are also mainly classified as ectothermic, but they do not encompass the entire group mentioned in the correct answer, which focuses specifically on reptiles and amphibians. Therefore, C correctly identifies the primarily ectothermic group within the options presented.