Final answer:
The odds ratio approximates the risk associated with a given exposure well when the frequency of the disease in the population is small. Therefore, the correct answer is c. The frequency of the disease in the population is small.
Step-by-step explanation:
The condition that allows the odds ratio to provide a good approximation of the risk associated with a given exposure is when the frequency of the disease in the population is small. Therefore, the correct answer is c. The frequency of the disease in the population is small. When the disease or event is rare, the odds ratio closely approximates the relative risk because the odds of the event in exposed and non-exposed individuals tend to be very similar to the actual probabilities.
Other factors, such as whether the controls are representative of the target population and whether the cases are typical with respect to severity and diagnostic criteria, while important for the validity of the study, do not directly influence the degree to which the odds ratio approximates the relative risk in the manner that disease frequency does.