Which of the following is a defining feature of amphibians?

The defining feature of amphibians is that they are cold-blooded vertebrates. This characteristic means that amphibians rely on external environmental temperatures to regulate their body heat, which categorizes them alongside reptiles and fish, but distinguishes them from warm-blooded animals such as mammals and birds, which maintain a constant internal body temperature regardless of external conditions.

Additionally, amphibians typically have a life cycle that includes both aquatic and terrestrial stages, often beginning as larvae in water before undergoing metamorphosis to become adult forms that may live on land. This duality, combined with their cold-blooded nature, allows amphibians to inhabit a wide range of environments. While they are primarily known for their association with water, they are not exclusively aquatic animals, as many adults spend time on land. Therefore, cold-blooded vertebrates accurately encapsulates a key aspect of amphibian biology.