**Core Concept**
Demography is the branch of biostatistics that specifically focuses on the study of human populations, including their size, structure, and dynamics. This encompasses the analysis of vital events such as births, deaths, and marriages.

**Why the Correct Answer is Right**
Demography involves the collection and analysis of data on demographic events to understand population trends and patterns. It uses statistical methods to describe and predict population growth, mortality rates, fertility rates, and other demographic indicators. Demographers often rely on census data, vital statistics, and other sources to inform their research.

**Why Each Wrong Option is Incorrect**
**Option A:** Epidemiology is a branch of biostatistics that deals with the study of the distribution and determinants of diseases in populations. While epidemiology may use demographic data, it is not primarily concerned with demographic events.
**Option B:** Biometry is a branch of biostatistics that deals with the application of statistical methods to biological and medical research. Biometry may involve the analysis of demographic data, but it is a broader field that encompasses many areas of biostatistics.
**Option C:** Actuarial science is a field that applies statistical methods to the analysis of risk and uncertainty in finance and insurance. Actuarial science may involve the use of demographic data, but it is not a branch of biostatistics.

**Clinical Pearl / High-Yield Fact**
One key aspect of demography is the concept of the "demographic transition," which describes the shift from high birth rates and high death rates (characteristic of pre-industrial societies) to low birth rates and low death rates (characteristic of developed societies).

**Correct Answer: D. Demography. Demography.**