Domestic animals are animals that are tamed and kept by humans for various purposes. These animals have been domesticated over the years and have adapted to living with humans. They provide companionship, help with work, and serve as a source of food and materials.

Among the options given, the correct answer is option 'B' - Horse.

Here is a detailed explanation of why horse is considered a domestic animal:

1. Definition of a domestic animal:
- Domestic animals are animals that have been selectively bred and tamed by humans for various purposes.
- These animals have undergone generations of domestication and have adapted to living in close proximity to humans.

2. Characteristics of domestic animals:
- Domestic animals are typically docile and can be easily trained and controlled by humans.
- They have undergone genetic changes through selective breeding, making them different from their wild counterparts.
- They are used for various purposes such as transportation, agriculture, companionship, and food.

3. Domestication of horses:
- Horses were one of the first animals to be domesticated by humans.
- The domestication of horses dates back thousands of years, and they have played a significant role in human history.
- Horses were initially used as a means of transportation, allowing humans to travel more efficiently and carry heavy loads.
- They were also used in agriculture for plowing fields and pulling carts.
- Over time, horses have become an integral part of various activities, including sports, recreation, and therapy.

4. Adaptation to domestication:
- Domesticated horses have undergone significant physical and behavioral changes compared to their wild ancestors.
- They have been selectively bred for specific traits such as size, strength, speed, and temperament.
- Domestic horses have adapted to living in close proximity to humans and being handled and ridden by them.
- They have also developed a social bond with humans and can form strong relationships with their owners.

In conclusion, among the given options, the horse is considered a domestic animal. Horses have been selectively bred and tamed by humans for various purposes, and they have adapted to living in close proximity to humans. They have played a significant role in human history and continue to be used for transportation, agriculture, companionship, and various other activities.