If there is an option for Vitamin B12 then that will be your answer.
I think the anwser is letter b
Answer: observational study
Explanation:
The observational studies are those studies where the researchers observes the samples or individuals of research without any intervention or manipulation.
In epidemiology, and nutritional studies the researcher observes the risk factors associated with the disease, diagnosis and treatment. The effect of a diet on the patient or volunteers can be done through observational studies.
