**Core Concept**
The question is testing the understanding of epidemiological terms, specifically the definition of a new case in a population. In epidemiology, the number of new cases occurring in a defined population during a specified period of time is a crucial measure for assessing the incidence of a disease.

**Why the Correct Answer is Right**
The correct answer refers to the incidence rate, which is a measure of the number of new cases of a disease occurring in a population over a specified period of time. Incidence rate is expressed as the number of new cases per unit of population per unit of time, usually per 1,000 or 100,000 people per year. The incidence rate is an important measure for understanding the burden of a disease and for evaluating the effectiveness of disease prevention and control strategies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the term "prevalence" refers to the total number of cases of a disease in a population at a given time, not the number of new cases occurring over a specified period of time.

**Option B:** This option is incorrect because the term "mortality rate" refers to the number of deaths from a disease or cause in a population over a specified period of time, not the number of new cases.

**Option C:** This option is incorrect because the term "case fatality rate" refers to the proportion of people with a particular disease who die from that disease, not the number of new cases occurring over a specified period of time.

**Clinical Pearl / High-Yield Fact**
It is essential to distinguish between incidence and prevalence when evaluating the burden of a disease in a population. Incidence rate provides valuable information about the risk of developing a disease, while prevalence rate provides information about the total number of cases in the population.

**Correct Answer:** C. Incidence rate.