Explanation:

- The study of population and its dynamics involves understanding how populations change over time, including factors like birth rates, death rates, and migration.

- Option 1: Democracy

- Refers to a system of government by the whole population, usually through elected representatives.

- Not related to the study of population dynamics.

- Option 2: Demography

- The scientific study of populations, focusing on the structure, dynamics, and trends.

- Directly related to the given group of words.

- Option 3: Dysgraphia

- A learning disability that affects writing abilities.

- Unrelated to population studies.

- Option 4: Dermatology

- The branch of medicine dealing with the skin and its diseases.

- Not connected to population dynamics.