Final answer:
There are five sacral vertebrae fused to form the sacrum, making it a single triangular-shaped bone part of the vertebral column.
Step-by-step explanation:
The sacrum is a single bone located near the inferior end of the adult vertebral column and is formed by the fusion of five sacral vertebrae. This fusion process, which creates a single triangular-shaped bone, does not begin until after the age of 20 and continues into middle age. The vertebral column itself consists of several regions: seven cervical vertebrae, twelve thoracic vertebrae, five lumbar vertebrae, the sacrum, and the coccyx. Therefore, the correct answer to how many sacral vertebrae are fused to form the sacrum is option c) five.