Answer:
The correct answer is option a. "Cross-sectional study".
Step-by-step explanation:
A cross-sectional study is a type of observational study used in medical and social researches. Cross-sectional studies are characterized by the analysis of data that comes from a population at a specific point of time. A research that involves studying the incidence of cholera carriers at three different villages with three different types of water supply, is an example of a cross-sectional study.