## Core Concept
The term "incidence" is a crucial concept in epidemiology, referring to the rate of new cases of a disease or condition that occur within a specified period among a population at risk.

## Why the Correct Answer is Right
The correct definition of incidence is the number of **new cases** that develop in a specified time period among a population at risk. This measure is essential for understanding the rate at which a disease is occurring in a population. It helps in assessing the risk of developing a particular condition and in planning healthcare services.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately define incidence. Without the specific details of option A, we can infer that any definition not focusing on "new cases" within a specified period among a population at risk would be incorrect.
* **Option B:** Similarly, this option is incorrect for the same reason; it likely does not specify "new cases" or might confuse incidence with prevalence.
* **Option D:** This option is also incorrect as it does not provide the correct definition of incidence.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **incidence rate** is usually expressed as the number of cases per 1,000 or 100,000 population per year. This measure is critical in tracking the occurrence of diseases over time and in evaluating the effectiveness of preventive measures.

## Correct Answer: C. Number of new cases that develop in a specified time period among a population at risk.