**Core Concept**
A case-control study is a type of observational study used to investigate the association between a particular exposure and a specific outcome or disease. In this study design, researchers select individuals who have the outcome of interest (cases) and compare them to individuals who do not have the outcome (controls). The goal is to identify potential risk factors or exposures that may contribute to the development of the disease.

**Why the Correct Answer is Right**
The risk in a case-control study is calculated using the odds ratio (OR), which is a measure of association between an exposure and an outcome. The OR is calculated by comparing the odds of exposure among cases to the odds of exposure among controls. This is expressed mathematically as: OR = (a/b) / (c/d), where a is the number of exposed cases, b is the number of unexposed cases, c is the number of exposed controls, and d is the number of unexposed controls. The OR provides an estimate of the strength and direction of the association between the exposure and the outcome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the risk in a case-control study is not calculated using the relative risk (RR), which is a measure of absolute risk difference between exposed and unexposed groups.
**Option B:** This option is incorrect because the risk in a case-control study is not calculated using the attributable risk (AR), which is a measure of the proportion of disease cases that can be attributed to a specific exposure.
**Option C:** This option is incorrect because the risk in a case-control study is not calculated using the number needed to harm (NNH), which is a measure of the number of individuals who need to be exposed to a treatment or intervention in order to cause one additional harmful event.

**Clinical Pearl / High-Yield Fact**
Remember that case-control studies are retrospective in nature and are useful for identifying potential risk factors or exposures that may contribute to the development of a disease. However, they are limited by their reliance on recall bias and the potential for confounding variables.

**Correct Answer: C. Odds Ratio. The odds ratio (OR) is a measure of association between an exposure and an outcome in a case-control study.**