Answer:

In order to determine which set consists of only mammals, we need to understand what mammals are and identify the animals in each set. Mammals are a group of animals that have certain characteristics:

1. Warm-blooded: Mammals have a constant body temperature, which means their bodies do not change temperature with the environment.

2. Have fur or hair: Mammals have some form of fur or hair on their bodies, even if it is very fine or sparse.

3. Breathe air: Mammals have lungs and breathe in oxygen from the air.

4. Bear live young: Mammals give birth to live young that are nourished with milk produced by the mother.

Now let's analyze each set of animals to identify the mammals:

a) Lion, hen, and human beings
- Lion: Lions are mammals as they have fur, are warm-blooded, breathe air, and give birth to live young.
- Hen: Hens are birds, not mammals, as they have feathers, lay eggs, and have a beak.
- Human beings: Humans are mammals as they have fur (hair), are warm-blooded, breathe air, and give birth to live young.

From set a), we can see that only one out of the three animals (lion) is a mammal. Therefore, this set does not consist of only mammals.

c) Tigers, sheep, and crocodiles
- Tigers: Tigers are mammals as they have fur, are warm-blooded, breathe air, and give birth to live young.
- Sheep: Sheep are mammals as they have fur, are warm-blooded, breathe air, and give birth to live young.
- Crocodiles: Crocodiles are reptiles, not mammals, as they have scales, lay eggs, and have a snout.

From set c), we can see that two out of the three animals (tigers and sheep) are mammals. Therefore, this set does not consist of only mammals.

d) Birds, butterflies, and frogs
- Birds: Birds are not mammals as they have feathers, lay eggs, and have a beak.
- Butterflies: Butterflies are insects, not mammals, as they have six legs, wings, and a proboscis.
- Frogs: Frogs are amphibians, not mammals, as they have moist skin, lay eggs in water, and undergo metamorphosis.

From set d), we can see that none of the animals are mammals. Therefore, this set does not consist of only mammals.

b) Lions, goats, and whales
- Lions: Lions are mammals as they have fur, are warm-blooded, breathe air, and give birth to live young.
- Goats: Goats are mammals as they have fur, are warm-blooded, breathe air, and give birth to live young.
- Whales: Whales are mammals as they have fur (blubber), are warm-blooded, breathe air, and give birth to live young.

From set b), we can see that all of the animals (lions, goats, and whales) are mammals. Therefore, this set consists of only mammals.

Therefore, the correct answer is option B (Lions, goats, and whales).