The largest and fastest bird in the world is the ostrich.

Size:
- The ostrich is the largest bird in the world, with males standing up to 9 feet tall and weighing around 250-350 pounds.
- Their long necks and legs, along with their large bodies, contribute to their impressive size.

Speed:
- Ostriches are also the fastest running birds, capable of reaching speeds of up to 60 miles per hour.
- Their powerful legs and long strides allow them to cover large distances quickly.
- They can maintain a speed of around 30-40 miles per hour for extended periods.

Physical Characteristics:
- Ostriches have unique physical characteristics that contribute to their size and speed.
- They have long, strong legs with only two toes, which help them run at high speeds.
- Their wings are large and strong, but they are primarily used for balance and courtship displays rather than flying.
- Ostriches have a long neck and a small head with large eyes and a sharp beak.
- Their bodies are covered in feathers, which are mainly black and white, providing camouflage in their natural habitat.

Habitat and Diet:
- Ostriches are native to the African savannas and deserts, where they inhabit open grasslands and semi-arid areas.
- They are well-adapted to these environments and can withstand high temperatures and lack of water.
- Ostriches are omnivores and feed on a variety of vegetation, seeds, insects, and small vertebrates.

Behavior:
- Ostriches are social birds that live in groups called flocks.
- They have a hierarchical social structure, with dominant males leading the group and defending their territory.
- During courtship, males perform elaborate displays, such as spreading their wings and tail feathers and shaking their heads, to attract females.

Conclusion:
In conclusion, the ostrich is the largest and fastest bird in the world. Their impressive size and speed are attributed to their unique physical characteristics, such as their long legs and powerful wings. Ostriches are well-adapted to their natural habitat and exhibit interesting behaviors.

I think (a) Ostrich