Which bone is known to form a joint with the pelvis?

The bone that forms a joint with the pelvis is the sacrum. The sacrum is a triangular-shaped bone located at the base of the spine and consists of five fused vertebrae. It connects the spine to the pelvis, forming the sacroiliac joint with the ilium of the pelvis on both sides. This joint plays a crucial role in supporting the weight of the upper body when standing and walking, as well as providing stability and movement between the spinal column and the pelvis.

The coccyx, commonly known as the tailbone, is located below the sacrum and does not form a joint with the pelvis in the same manner. The lumbar vertebrae are part of the lower back but do not directly connect to the pelvis. The pubis, while it is part of the pelvic girdle, does not serve as a bone that directly forms a joint with the pelvis in the way that the sacrum does. Therefore, the correct answer highlights the important role of the sacrum in the anatomy and function of the pelvic region.