**Core Concept**
Spalding sign is a radiological feature observed in the skull, indicating an increase in intracranial pressure due to a space-occupying lesion. It is characterized by a thinning of the inner table of the skull with associated cortical erosion, leading to a characteristic 'hair-on-end' appearance on radiographs.

**Why the Correct Answer is Right**
The Spalding sign is a result of the increased intracranial pressure causing the bone to be stretched and thinned, particularly in the areas where the bone is thinnest. This thinning is most pronounced over the inner table of the skull, leading to cortical erosion. The 'hair-on-end' appearance on radiographs is due to the perpendicular orientation of the thinned cortical bone to the X-ray beam.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the Spalding sign is not typically associated with cranial trauma. While trauma can cause skull fractures and associated changes, the Spalding sign is a specific indicator of increased intracranial pressure.
**Option B:** This option is incorrect as the Spalding sign is not a feature of normal skull anatomy. The 'hair-on-end' appearance is a pathognomonic sign of increased intracranial pressure.
**Option C:** This option is incorrect as while osteoporosis can cause thinning of the skull, it does not result in the characteristic 'hair-on-end' appearance seen in the Spalding sign.

**Clinical Pearl / High-Yield Fact**
The Spalding sign is a classic radiological feature of increased intracranial pressure, and its presence should prompt further investigation into the underlying cause. It is essential to recognize this sign to avoid missing a potentially life-threatening condition.

**Correct Answer:** C. Osteosarcoma is the condition in which Spalding sign is seen.