ANSWER: CASE-CONTROL STUDY
EXPLANATION:
An epidemiology expert will make use of a Case-control Study for proper determination of association or linkage between the oil spill and the disease outbreak (arthritis).
Case-control Study provides an observational experiment between two existing groups (two reservoirs) differing in outcome (affected and not affected), they are highlighted and compared on the basis of a context (disease outbreak; arthritis).
However, the known group is regarded as the control group and it provides a substantial guide to the confirm the validity of other experimental groups in relation to the general context (outbreak of arthritis).