**Core Concept**
The pelvis is the most reliable bony structure for sex determination in forensic medicine due to its distinct morphological differences between males and females, especially in the shape and size of the pelvic inlet and outlet.

**Why the Correct Answer is Right**
The pelvis exhibits pronounced sexual dimorphism, with females having a wider, shallower pelvic inlet (gravid pelvis) and a broader, more circular shape to accommodate childbirth. Males have a narrower, deeper inlet and a more robust, angular structure. These differences are consistently visible on radiographic or anatomical examination and are more pronounced than in other bones like the skull or femur. The pelvis is thus the most accurate and commonly used site for sex determination in forensic cases.

**Why Each Wrong Option is Incorrect**
Option A: The skull shows some sex-related differences, such as mastoid process size and brow ridge prominence, but these are less reliable and can vary significantly due to individual variation or trauma.
Option B: The femur is long and slender in both sexes, with minimal sexual dimorphism, making it less useful for accurate sex determination.
Option D: The tibia is similar in size and shape between sexes and lacks consistent morphological differences for sex determination.

**Clinical Pearl / High-Yield Fact**
Always examine the pelvis first in forensic skeletal analysis—its shape is the most reliable indicator of sex, especially in cases where soft tissue is absent or damaged.

✓ Correct Answer: C. Pelvis