**Core Concept**
The sacrum develops from five separate sacral bones during embryogenesis, which gradually fuse as a person matures. This fusion is a key event in skeletal development, marking the transition from a segmented to a unified bony structure in the pelvis.

**Why the Correct Answer is Right**
By the age of 25, the five sacral vertebrae typically fuse completely into a single, solid bone—the sacrum. This fusion is a well-documented process in human skeletal development, driven by ossification and growth plate closure. The process begins early in life but completes around 25 years, making this a key milestone in forensic age estimation, especially in postmortem skeletal analysis.

**Why Each Wrong Option is Incorrect**
Option A: 15 – At 15, the sacral bones have not fully fused; fusion is still incomplete and typically not complete by this age.
Option C: 30 – Fusion is already complete by 25; 30 is too late and not representative of the typical completion time.
Option D: 40 – By 40, fusion is already complete, but this age is not clinically significant for sacral fusion and overestimates the timeline.

**Clinical Pearl / High-Yield Fact**
In forensic medicine, the fusion of the sacrum is a key indicator of skeletal maturity. A fused sacrum at age 25 is used to estimate age in adult skeletal remains, especially when combined with other ossification markers like the epiphyses of long bones.

✓ Correct Answer: B. 25