**Core Concept**
The cephalic index is a quantitative measure used in physical anthropology to classify the skull shape and size of an individual based on the ratio of the maximum width of the head to its maximum length. It is calculated by dividing the maximum width of the head by the maximum length and multiplying the result by 100.

**Why the Correct Answer is Right**
The cephalic index is most useful in identifying Mongoloid populations, which are characterized by a long and narrow skull shape, typically with a cephalic index between 70 and 79. This is due to the genetic adaptation of these populations to their environment, which led to the development of a more elongated skull shape. The cephalic index is also useful in distinguishing between different populations, such as Caucasoids, Negroids, and Australoids, each with distinct skull shapes and sizes.

**Why Each Wrong Option is Incorrect**
**Option A:** The cephalic index is not most useful in identifying Australoids, as their skull shape is more variable and does not fit neatly into the cephalic index classification.
**Option B:** The cephalic index is not primarily used to identify Caucasoids, as they exhibit a wide range of skull shapes and sizes, making the cephalic index less useful for classification.
**Option C:** The cephalic index is not most useful in identifying Negroids, as their skull shape is generally more rounded and does not fit well into the cephalic index classification.

**Clinical Pearl / High-Yield Fact**
The cephalic index is an important tool in physical anthropology, but it should be used with caution, as it can be influenced by various factors, such as age, sex, and nutrition, which can affect skull shape and size.

**Correct Answer: D. Mongoloid populations.**