Answer:
Option A, An observational study because it involves the pre-existing condition of the subjects
Step-by-step explanation:
In an observational study, a researcher's aim is to observe the effect of an unusual condition, medicine, treatment, or any other intervention on the subject of the experiment without differentiating whether the subject was ever exposed to any such condition or not. Also, this kind of study involves study of pre-exposure condition as well thus, it becomes easy to identify what are the impacts of any treatment/medicine/unusual condition.
Hence, option A is correct.