**Core Concept**
The secondary attack rate (SAR) is a measure used in epidemiology to calculate the number of secondary cases that occur among the contacts of an index case within a specified time period. It is expressed as a percentage and is used to assess the contagiousness of an infectious disease.

**Why the Correct Answer is Right**
The denominator in the calculation of SAR is the total number of people who were exposed to the index case and were at risk of developing the disease. This includes all individuals who were in close contact with the index case and were susceptible to the disease. The SAR is calculated by dividing the number of secondary cases by the total number of people exposed and at risk. The correct denominator should exclude individuals who were not exposed or were not susceptible to the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct population at risk.
* **Option B:** This option is incorrect because it does not account for individuals who were not exposed or were not susceptible to the disease.
* **Option C:** This option is incorrect because it includes individuals who were not exposed to the index case.

**Clinical Pearl / High-Yield Fact**
When calculating the secondary attack rate, it is essential to clearly define the population at risk and exclude individuals who were not exposed or were not susceptible to the disease.

**Correct Answer: C. The total number of people who were exposed to the index case and were at risk of developing the disease.**