Final answer:
The sacral portion of the spine has one bone, the sacrum, which is formed by the fusion of five sacral vertebrae.
Step-by-step explanation:
The sacral portion of the spine consists of a single bone called the sacrum in adult humans. This bone is located near the inferior end of the vertebral column and is formed by the fusion of five sacral vertebrae. The process of these vertebrae fusing into one bone does not begin until after the age of 20 and is part of the posterior portion of the pelvis. The vertebral column plays a critical role in surrounding and protecting the spinal cord, supporting the head, and serving as an attachment point for ribs and back muscles.