Which bone is primarily responsible for forming the back wall of the skull?

The occipital bone is primarily responsible for forming the back wall of the skull. It is a large, wedge-shaped bone located at the lower back part of the cranium. One of its critical features is the foramen magnum, which is the large opening at the base of the skull that allows the spinal cord to connect with the brain. The occipital bone also provides attachment points for various muscles and ligaments that support the head and neck. Its positioning and structure make it essential for protecting the brain and contributing to the overall architecture of the skull.

In contrast, the temporal bone is situated on the sides of the skull and does not contribute to the back wall, while the sphenoid bone is located more towards the center of the skull and helps form the base. The ethmoid bone, being a delicate bone found in the skull's anterior region, also does not play a role in constituting the back wall of the skull. Thus, the occipital bone's specific location and structural characteristics establish it as the correct answer.