All of the following are Experimental / Interventional studies except:: (A) Randomised control trials ., (B) Field trials., (C) Community trials., (D) Ecological studies.
Answer is D. Ecological study (Correlational study):

Type of analytical (observational) epidemiological study which provide the ‘least satisfactory type of evidence on causality’
Units of study: Population
Advantage: Data can be used from populations with different characteristics
Potential problem: Socio-economic confounding
Ecological fallacy: Is an error of interpretation of statistical data in an ecological study, whereby characteristics are ascribed to a group of individuals which they may not possess as individuals