Which part of the spinal column is formed by fused bones?

The sacrum is formed by fused bones, specifically the fusion of five vertebrae that create a single triangular-shaped structure at the base of the spine. This fusion provides stability and strength to the pelvic region, forming the posterior part of the pelvis and connecting the spine to the hips. The sacrum plays a critical role in supporting the upper body's weight while standing and during movement.

In contrast, the cervical, lumbar, and thoracic regions of the spinal column consist of individual vertebrae that are not fused. The cervical region contains seven vertebrae, the thoracic region has twelve, and the lumbar region has five, allowing for a range of motion and flexibility not found in the fused sacral bones. Thus, the correct answer reflects the unique structural feature of the sacrum as a segment of the spinal column made up of fused vertebrae.