**Core Concept**
The cross product ratio, also known as the odds ratio (OR), is a statistical measure used to quantify the association between an exposure and an outcome in a case-control study. It represents the ratio of the odds of exposure among cases to the odds of exposure among controls.

**Why the Correct Answer is Right**
The cross product ratio is used as an estimate of the relative risk (RR) in case-control studies, where the outcome is rare. This is because the RR is not directly calculable from the case-control study design. The cross product ratio is calculated as the product of the sensitivity and specificity of the exposure, which in turn are calculated from the true positives, true negatives, false positives, and false negatives in the study. The cross product ratio is a useful measure for assessing the association between an exposure and an outcome in case-control studies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the cross product ratio is not used in cohort studies, which are designed to estimate the incidence of an outcome in relation to an exposure.
**Option B:** This option is incorrect because the cross product ratio is not used in cross-sectional studies, which are designed to estimate the prevalence of an outcome in relation to an exposure.
**Option C:** This option is incorrect because the cross product ratio is not used in randomized controlled trials (RCTs), which are designed to estimate the efficacy of an intervention.

**Clinical Pearl / High-Yield Fact**
Remember that the cross product ratio is a useful measure for assessing the association between an exposure and an outcome in case-control studies, but it should be interpreted with caution due to its dependence on the study design and the potential for bias.

**Correct Answer: D. Case-control studies**