**Core Concept**
The ratio of incidence among exposed and non-exposed individuals is a fundamental measure in epidemiology, used to quantify the association between a risk factor and a disease. This ratio is essential for understanding the strength and direction of the relationship between the exposure and the outcome.

**Why the Correct Answer is Right**
The correct answer is the **Relative Risk (RR)**, which is calculated as the ratio of the incidence of the disease among exposed individuals to the incidence among non-exposed individuals. This measure indicates the degree to which the exposure increases the risk of developing the disease. For example, if the RR is 2, it means that the exposed individuals are twice as likely to develop the disease as the non-exposed individuals.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Attributable Risk (AR)** is the difference in incidence between exposed and non-exposed individuals, not the ratio.
* **Option B:** **Odds Ratio (OR)** is a measure of association between an exposure and an outcome, but it is not the ratio of incidence among exposed and non-exposed individuals.
* **Option D:** **Risk Difference (RD)** is the difference in risk between exposed and non-exposed individuals, not the ratio.

**Clinical Pearl / High-Yield Fact**
Remember the difference between Relative Risk (RR) and Odds Ratio (OR): RR is a direct measure of the risk of developing a disease, while OR is an indirect measure that requires careful interpretation.

**Correct Answer: A. Relative Risk (RR)**