Answer:
Epidemiological Study
Step-by-step explanation:
An epidemiological study is a study that examines the degree to which an exposed population or sample prone to illness or death compared to the degree to which a similar unexposed population or sample are prone to illness or death. This is done by considering sex, social status, age and race of the population.
Since the research group in 2018 identify a group of people who were exposed to contaminated drinking water during an incident in 1985 and another group who were never exposed in their study, it their type of study is an epidemiological study.