**Core Concept**
The identification of sex from adult skeletal remains is based on the presence and characteristics of secondary sex characteristics, which develop during puberty. These characteristics are influenced by sex hormones, such as testosterone and estrogen, and can be used to distinguish between males and females.

**Why the Correct Answer is Right**
The best skeletal element for sex determination is the pelvis, specifically the subpubic angle and the shape of the pelvic inlet. The subpubic angle is typically wider in females (average 90-100°) due to the wider pelvic outlet and the presence of a more pronounced pubic bone. In contrast, the subpubic angle is narrower in males (average 60-80°). This difference is due to the larger size and more robust construction of the male pelvis, which is adapted for childbirth and the support of the fetus. The pelvic inlet is also more oval in shape in females, with a wider transverse diameter, whereas in males, it is more circular.

**Why Each Wrong Option is Incorrect**
* **Option A:** The skull is not a reliable indicator of sex, as the differences between males and females are relatively minor and can be influenced by various factors, such as age and ancestry.
* **Option B:** The long bones, such as the femur and humerus, do not provide reliable information about sex, as they are similar in both males and females.
* **Option C:** The sternum is not a useful indicator of sex, as it is relatively small and can be affected by various factors, such as age and disease.

**Clinical Pearl / High-Yield Fact**
When examining the pelvis for sex determination, it is essential to note that the subpubic angle and the shape of the pelvic inlet can be affected by various factors, such as age, disease, and trauma. Therefore, it is crucial to consider the overall morphology of the pelvis and to look for multiple indicators of sex before making a determination.

**Correct Answer: C. The pelvis**