Introduction:
The question asks which of the given options can live both in water and on land. Let's analyze each option to determine the correct answer.

Analysis of options:

a) Frog:
Frogs are amphibians, which means they can live both in water and on land. They have specialized adaptations that allow them to survive in both environments. As tadpoles, they live in water and breathe through gills. However, as they mature into adults, they develop lungs and can breathe air. Frogs also have webbed feet, which help them swim in water, as well as strong hind legs for hopping on land. Therefore, the frog can live in both aquatic and terrestrial habitats.

b) Fish:
Fish are exclusively aquatic animals. They have gills that extract oxygen from water, allowing them to breathe underwater. Although some fish can briefly survive out of water, they rely on water for their survival. Therefore, fish cannot live on land.

c) Dog:
Dogs are terrestrial animals and cannot live in water for extended periods. While they may be able to swim, they are not adapted to survive underwater. Dogs are mammals and have lungs, which require them to breathe air. They do not have the necessary adaptations to live in aquatic habitats.

d) Bird:
Birds are primarily terrestrial animals, but some species are capable of swimming and diving in water. However, they are not adapted to live exclusively in aquatic environments. Birds have feathers that provide insulation and allow them to fly, but they are not suited for swimming like the webbed feet of aquatic animals. Therefore, while some birds can spend time in water, they cannot live exclusively in water.

Conclusion:
Based on the analysis, the correct answer is option 'a' - Frog. Frogs are amphibians that have adaptations enabling them to live both in water and on land. They can breathe both underwater and in the air and have physical features that allow them to swim and hop on land.