Which of the following is an example of an amphibian?

An amphibian is defined as a member of the class Amphibia, which typically includes animals that undergo a transformation from a juvenile aquatic stage with gills to an adult stage with lungs. Frogs exemplify this life cycle perfectly, starting as tadpoles in water and then maturing into adult frogs that can live on land as well as in water. This transition between aquatic and terrestrial environments is a hallmark characteristic of amphibians.

In contrast, goldfish are classified as fish within the class Actinopterygii, and they do not undergo such a transformation; they remain aquatic throughout their lives. Chickens belong to the poultry category and are birds, part of the class Aves, which lay eggs but do not have an amphibious life cycle. Squirrels, being mammals under the class Mammalia, are warm-blooded, fur-covered creatures that give live birth and do not exhibit any amphibious traits. Therefore, the only option that fits the criteria for amphibians is the frog.