Introduction:
The goldfinch bird is known for its vibrant golden plumage and beautiful song. Like many other birds, goldfinches build nests to lay their eggs and raise their young. These nests are usually constructed in trees or shrubs, providing a safe and secure environment for the birds and their offspring.

Explanation:
The young ones of the goldfinch bird can be found on the thickness of the branch. Here's a detailed explanation of why the correct answer is option 'A':

1. Nest Location:
Goldfinches typically build their nests in trees or shrubs, using materials such as twigs, grass, and moss. These nests are usually situated on the branches, providing a stable foundation for the nest structure. The location of the nest ensures that it is well-hidden from predators and provides a comfortable space for the young ones.

2. Branch Thickness:
The young goldfinches prefer to stay on the thickness of the branch. This is because the thicker branches offer better stability and support compared to thinner branches. The young birds are still developing their muscles and coordination, so they need a secure surface to perch on without the risk of falling or losing their balance.

3. Safety and Protection:
By staying on the thickness of the branch, the young goldfinches are better protected from potential predators. The thicker branches provide more cover and make it more difficult for predators to reach the young birds. This increases their chances of survival during the vulnerable stage of their life.

4. Feeding and Learning:
The young goldfinches also benefit from staying on the thickness of the branch as it gives them easy access to food sources. Goldfinches primarily feed on seeds, especially those from thistle plants. By perching on the branch, the young birds can easily reach the seed heads and learn how to extract and consume the seeds.

5. Parental Care:
The adult goldfinches diligently care for their young ones. They provide food, protection, and guidance during the early stages of their development. By staying on the thickness of the branch, the young birds are within close proximity to their parents, making it easier for the adults to attend to their needs.

Conclusion:
In conclusion, the young ones of the goldfinch bird can be found on the thickness of the branch. This location provides safety, stability, and access to food sources, ensuring the well-being and development of the young goldfinches.