**Core Concept**
Ashley's rule is a method used in forensic anthropology to estimate the sex of an individual based on the size and shape of the sternum. This technique relies on the differences in sternum morphology between males and females, which can be used to infer the sex of an individual.

**Why the Correct Answer is Right**
Ashley's rule takes into account the dimensions of the sternum, including its length, width, and depth. Specifically, the rule states that in males, the length of the sternum is greater than or equal to the length of the 4th costal cartilage, whereas in females, the length of the sternum is less than the length of the 4th costal cartilage. This difference in sternum morphology is a reliable indicator of sex.

**Why Each Wrong Option is Incorrect**
**Option A:** Ashley's rule is not used to determine age using the sternum. While the sternum can provide clues about an individual's age, Ashley's rule is specifically focused on sex determination.

**Option C:** Ashley's rule is not used to determine age using the mandible. The mandible is a separate bone that can be used to estimate age, but Ashley's rule is specifically related to the sternum.

**Option D:** Ashley's rule is not used to determine sex using the mandible. While the mandible can provide clues about an individual's sex, Ashley's rule is specifically focused on the sternum.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Ashley's rule is a useful tool in forensic anthropology, but it should be used in conjunction with other methods to ensure accurate sex determination. Additionally, the accuracy of Ashley's rule can be influenced by various factors, including the preservation of the sternum and the presence of any trauma or disease.

**✓ Correct Answer: B. Determine sex using sternum**