Step 1: Understand the field of Demography.
Demography is the scientific study of human populations, focusing on the structure, dynamics, and trends of populations over time.
Step 2: Review other options.
- Geography studies the Earth's surface, but does not focus specifically on human population.
- Geology studies the Earth's physical structure and its history, not populations.
- Biology is the study of life forms, but it is not exclusively concerned with human populations.
Step 3: Conclusion.
Thus, the correct answer is ((D) Demography, as it is the specific field of study for human population.
Final Answer:
\[
\boxed{((D) \; Demography}
\]