Correct Answer: Amphibian

Solution : The correct answer is the second option.

The word amphibian is the most appropriate one-word substitute for the group of words because it specifically refers to animals that can live both on land and in water. Amphibians are a class of vertebrates that includes animals like frogs, toads, crocodiles, etc. that have adaptations that allow them to thrive in both terrestrial and aquatic environments.

The meanings of the other options are as follows:

• Marine pertains to the sea and aquatic environments but does not imply land-dwelling.
• Arboreal refers to animals that primarily live in trees.
• Terrestrial describes animals that live on land but not necessarily in water.