**Core Concept**
The concept being tested is a fundamental principle in epidemiology, which is the study of the distribution and determinants of health-related events, diseases, or health-related characteristics among populations.

**Why the Correct Answer is Right**
The frequency of a disease or characteristic expressed per unit size of the population or group in which it is observed is known as the **incidence rate**. This measure is calculated as the number of new cases of a disease or characteristic that occur within a specified time period per unit size of the population, usually expressed as a rate per 1,000 or 100,000 individuals per year. The incidence rate is an important measure in epidemiology as it helps to quantify the risk of developing a disease or characteristic within a given population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the concept of expressing frequency per unit size of the population.
**Option B:** Prevalence is a measure of the total number of cases of a disease or characteristic in a population at a specific point in time, but it does not account for the size of the population.
**Option C:** This option is not a valid measure of expressing frequency per unit size of the population.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the key difference between incidence and prevalence is that incidence measures the rate of new cases, while prevalence measures the total number of cases in a population at a given time.

**Correct Answer: D. Incidence rate.**