Answer:
An epidemiological study.
Explanation:
Epidemiology may be defined as the sub field of immunology that mainly deals with the study of the particular disease especially in the group of population of human beings.
The epidemiology includes the main reason of the disease, its cause and its medication. In the given example, the scientists study about the heart rate incident in the population. Later, the regular effect of the fish oil is also noticed in the population. This study clearly indicates the epidemiological study.
Thus, the correct answer is epidemiological study.
