Here's a picture of all the phases:
Answer: a. Intervention study
Explanation:
In an interventional research study the participants receive an intervention as they are given a new item or commodity to use and the results of use are evaluated on the basis of change made by the commodity on the participant. Such studies are used for the purpose of clinical trails.
The given research design is the example of interventional study. As the margarine is a new commodity the ingredients of which are claimed to lower down the blood cholesterol levels which can tested by comparing the cholesterol levels in people.
