**Core Concept**
In a case-control study, researchers compare individuals with a specific outcome or disease (cases) to those without the outcome or disease (controls) to identify potential risk factors. This study design is often used to investigate rare diseases or outcomes.

**Why the Correct Answer is Right**
The risk ratio (also known as relative risk) is a measure of the strength of the association between an exposure and an outcome. It is calculated as the ratio of the probability of the outcome occurring in the exposed group to the probability of the outcome occurring in the unexposed group. In case-control studies, the risk ratio is estimated using the odds ratio (OR), which is a measure of association between an exposure and an outcome in a case-control study.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the risk ratio is not directly calculated in case-control studies due to the retrospective nature of the study design. The odds ratio is used as an estimate of the risk ratio.
**Option B:** This option is incorrect because the relative risk reduction is a measure of the proportion of the risk that is eliminated by exposure to a preventive measure, not a measure of association in a case-control study.
**Option C:** This option is incorrect because the attributable risk is a measure of the proportion of disease incidence that is attributable to a specific exposure, which is not directly calculated in case-control studies.

**Clinical Pearl / High-Yield Fact**
In case-control studies, the odds ratio is used as an estimate of the risk ratio due to the retrospective nature of the study design. However, the odds ratio can be biased if the exposure is rare or if there is a strong confounding effect.

**Correct Answer: C. The odds ratio is used as an estimate of the risk ratio in case-control studies.**