Which bone is located at the back of the skull?

The occipital bone is the correct answer as it is the bone situated at the back of the skull. This bone plays a crucial role in forming the base of the skull and supports the structure of the head. It contains the occipital condyles, which articulate with the first cervical vertebra, allowing for the nodding motion of the head. Its prominence and location are essential for protecting the brain and providing attachment points for neck muscles.

The other bones mentioned are not located at the back of the skull. The sphenoid bone is situated more centrally and is often described as the keystone of the cranial floor, while the ethmoid bone is located between the nasal cavity and the orbits of the eyes, and the frontal bone forms the forehead and the upper part of the eye sockets. Each of these bones has distinct functions and locations within the skull structure, which differentiates them from the occipital bone.