The correct answer is the backbone because the backbone holds our body upright and  makes it steady so that we can walk and do other activities.

The characteristic feature that helps human beings to walk upright is their backbone, which holds the body up vertically.

Explanation:
- Human beings have a unique skeletal structure that enables them to walk upright. This is due to several key features of their anatomy.

Backbone as a Support:
- The backbone, or the vertebral column, is a crucial component of the human skeletal system. It consists of a series of individual bones called vertebrae, stacked on top of each other. The backbone provides support and stability to the body, allowing humans to stand and walk upright.
- The vertebrae are specifically designed to hold the body in an upright position. They are arranged in a way that forms a gentle S-shaped curve, which helps distribute the body weight evenly along the spine. This curvature also acts as a shock absorber, reducing the impact on the body while walking or running.

Muscles and Ligaments:
- In addition to the backbone, the muscles and ligaments surrounding the spine play a crucial role in maintaining an upright posture. These structures provide stability and control the movement of the spine. Muscles such as the erector spinae, which run along the back, help support the spine and maintain an upright position. Ligaments connect the vertebrae together, providing additional support and preventing excessive movement.

Centre of Gravity:
- Another factor that contributes to humans' ability to walk upright is their center of gravity. The center of gravity is the point where the body's weight is evenly distributed. In humans, the center of gravity is located near the pelvis, which is the bony structure in the lower part of the abdomen. This positioning allows for better balance and stability while walking or standing.

Benefits of Walking Upright:
- Walking upright provides humans with several advantages. It frees up the hands for various tasks and allows for efficient use of tools and objects. It also improves the field of vision, as the eyes are positioned at the front of the head, providing better depth perception and the ability to see long distances. Additionally, walking upright conserves energy compared to other forms of locomotion, such as crawling or walking on all fours.

Conclusion:
- In conclusion, the characteristic feature that enables human beings to walk upright is their backbone, which provides support and stability to the body. This, combined with the muscles, ligaments, and the positioning of the center of gravity, allows humans to maintain an upright posture and engage in the efficient and energy-saving activity of walking.