Answer:
c. observational study
Explanation:
Researchers compare the rates of autism for children who did and did not receive the standard measles-mumps-rubella vaccine, to see if the vaccine might be responsible for autism in some children.
This study design in the above scenario is observational study because the researcher are obviously observing whether the vaccine is affecting the children. Note that Observational study is like experiment which attempts to understand the cause and effect relationships on a subject.