Answer:
Observational study
Explanation:
Given that scientists study 200 adults who have type 2 diabetes, and 200 adults who have similar characteristics but do not have the disease.
For 18 months, the researchers collect lifestyle information on all the study participants
This is an observational study as actual effect of type II diabetes on 200 adults and those who do not have were observed. There were no experiments conducted or any medicine external given. As such they were studied and results recorded
Hence an observational study