**Core Concept**
The sacrum and coccyx are two fused vertebrae that form the lower part of the vertebral column, and the type of joint connecting them is crucial for understanding their structure and function.

**Why the Correct Answer is Right**
The joint between the sacrum and coccyx is a synchondrosis, which is a type of cartilaginous joint. This is because the two bones are connected by a plate of hyaline cartilage, allowing for limited movement and flexibility in the region. The synchondrosis joint allows for slight movement between the two bones, which is necessary for accommodating the stresses and strains placed on the pelvic region.

**Why Each Wrong Option is Incorrect**
**Option A:** A synarthrosis joint would imply a fibrous joint, which is not the case here. Synarthrosis joints, such as the suture between the bones of the skull, are immovable and do not allow for any significant movement.
**Option B:** A synovial joint would be characterized by a space between the articulating bones, which is not present in the sacrum-coccyx joint. Synovial joints, such as the knee or elbow, have a synovial cavity filled with synovial fluid that reduces friction between the bones.
**Option C:** An amphiarthrosis joint would imply a cartilaginous joint with limited movement, but the sacrum-coccyx joint is a synchondrosis, which is a specific type of cartilaginous joint.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sacrum and coccyx are fused bones, which can sometimes be a source of pain or discomfort, especially if there is a fracture or other injury to the area. Understanding the type of joint between these bones can help in diagnosing and managing these types of conditions.

**Correct Answer: D. Synchondrosis**