**Core Concept**
The secondary attack rate is a measure used in epidemiology to estimate the contagiousness of a disease within a specific population. It is calculated by dividing the number of secondary cases by the number of exposed individuals (denominator) and then multiplying by 100.

**Why the Correct Answer is Right**
The correct denominator for calculating the secondary attack rate includes all individuals who were exposed to the primary case, regardless of whether they developed the disease. This is because the secondary attack rate aims to quantify the proportion of exposed individuals who go on to develop the disease. The exposed population serves as the denominator, as it represents the total number of individuals who were at risk of developing the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct group of individuals (exposed population) that should be included in the denominator.

**Option B:** This option is incorrect because it suggests that the denominator should only include individuals who developed the disease, which is not accurate.

**Option C:** This option is incorrect because it implies that the denominator should include only individuals who developed symptoms, which is not the correct definition of the secondary attack rate.

**Clinical Pearl / High-Yield Fact**
When calculating the secondary attack rate, it's essential to remember that the denominator includes all exposed individuals, regardless of whether they developed the disease. This is crucial for accurately estimating the contagiousness of a disease within a population.

**Correct Answer: C. Exposed population.**