**Core Concept**
The concept being tested here is the definition of the interval between a primary case and a secondary case in the context of infectious disease epidemiology. A primary case is the first individual in a chain of transmission, while a secondary case refers to individuals who contract the disease from the primary case. Understanding this concept is crucial in tracking the spread of infectious diseases and identifying potential outbreaks.

**Why the Correct Answer is Right**
The interval between a primary and secondary case is known as the "generation time" or "incubation period." This refers to the time between exposure to the pathogen and the onset of symptoms in the individual. The generation time is an essential factor in estimating the basic reproduction number (R0) of a disease, which represents the average number of secondary cases generated by a single primary case. By knowing the generation time, public health officials can better predict the spread of a disease and implement targeted interventions to control outbreaks.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a recognized term in infectious disease epidemiology.
**Option B:** This option is incorrect because it refers to the time between symptom onset in a primary case and the onset of symptoms in a secondary case, not the interval between exposure and symptom onset.
**Option C:** This option is incorrect because it is not a specific term used to describe the interval between a primary and secondary case.

**Clinical Pearl / High-Yield Fact**
When investigating an outbreak, it's essential to calculate the generation time to estimate the basic reproduction number (R0) and predict the spread of the disease. This information can help inform public health interventions, such as vaccination campaigns or contact tracing.

**Correct Answer: C. Secondary attack rate.**