Answer is skeletal system because it a group of bones which helps our body to stay upside.
We think that the muscular system also helps our body but, it supports our bones.
The main system which holds our body is Skeletal system.

The correct answer is option 'D': the skeletal system holds our body upright. The skeletal system is made up of bones, cartilage, ligaments, and tendons, which work together to provide support, protection, and movement to the body.

The skeletal system provides structural support to the body, allowing us to stand upright. Here is a detailed explanation of how the skeletal system accomplishes this:

1. Structure of the skeletal system:
- Bones: The human body has 206 bones that make up the skeletal system. These bones are connected to each other by joints.
- Cartilage: Cartilage is a flexible, rubbery tissue that covers the ends of bones and provides cushioning.
- Ligaments: Ligaments are strong bands of connective tissue that connect bones to each other, providing stability and strength.
- Tendons: Tendons are tough fibrous tissues that connect muscles to bones, allowing for movement.

2. Role of the skeletal system in upright posture:
- Axial skeleton: The axial skeleton, which includes the skull, spine, and ribcage, provides support and protection for vital organs. The spine, in particular, plays a crucial role in maintaining an upright posture.
- Appendicular skeleton: The appendicular skeleton, which includes the bones of the limbs and the girdles that connect them to the axial skeleton, supports the weight of the body and allows for movement.

3. Functions of the skeletal system:
- Support: The skeletal system provides a framework that supports the body and maintains its shape.
- Protection: Bones protect vital organs such as the brain, heart, and lungs from injury.
- Movement: The skeletal system works together with muscles to allow for movement. Muscles attach to bones via tendons and contract to produce movement at the joints.
- Storage: Bones store minerals such as calcium and phosphorus, which are essential for bodily functions.
- Blood cell production: The bone marrow, located within certain bones, produces red and white blood cells and platelets.

In conclusion, the skeletal system plays a vital role in holding our body upright. Its structure, including bones, cartilage, ligaments, and tendons, provides support and stability, allowing us to maintain an upright posture.