**Core Concept**
The cephalic index is a measure used in physical anthropology to classify the shape and size of the human skull. It is calculated by dividing the maximum width of the head by its maximum length, multiplied by 100 to express it as a percentage.

**Why the Correct Answer is Right**
The cephalic index is a key factor in determining the racial or ethnic origin of a skull. It helps in categorizing skulls into different types, such as dolichocephalic (long and narrow), mesocephalic (medium and broad), and brachycephalic (short and broad). The cephalic index is used to study the evolution and migration patterns of different human populations.

**Why Each Wrong Option is Incorrect**
**Option A:** The cephalic index is not used for determining the sex of a skull. The sex of a skull is typically determined by examining the size and shape of the nuchal crest, mastoid process, and the size of the foramen magnum.
**Option B:** The cephalic index is not used for determining the age of a skull. The age of a skull is typically determined by examining the degree of cranial sutures closure, tooth wear, and other osteological features.
**Option C:** The cephalic index is not used for determining the cause of death. The cause of death is typically determined by examining the skeletal remains for signs of trauma, disease, or other pathological conditions.

**Clinical Pearl / High-Yield Fact**
The cephalic index is a useful tool in forensic anthropology for determining the racial or ethnic origin of human remains. It is also used in paleoanthropology to study the evolution of the human skull.

**Correct Answer:** D. Determination of racial or ethnic origin of a skull.