c) A study of how disease rates vary among different population groups
Explanation:
Epidemiology is the systematic and scientific study of the distribution of the health-related issue in a specified population.
The Epidemiologists studies the disease and its cause in a population by the collection of the data which gives an insight in the distribution of the health issues in the population living from a small area like school to a country.
The epidemiological studies help the society by reducing the risk of the negative health outcomes, in spreading awareness about the health issues and in framing the health policies.
