Answer:
The correct option is A: Prospective Study
Step-by-step explanation:
A prospective study is sometimes referred to as a prospective cohort study. This is usually a study involving a group, in which the participants are enrolled for the study prior to them developing the outcome that is being studied. From the study in the question, the scientist enrolled 500 healthy adults, which means that when he enrolled them, they were healthy and had no issues whatsoever. It was after 6 years that he began to take their blood pressure, which is the outcome of the study.