**Core Concept**
Rubella, also known as German measles, is a highly contagious viral infection caused by the rubella virus. It primarily affects children, but its impact can be severe in pregnant women, leading to congenital rubella syndrome.

**Why the Correct Answer is Right**
The correct answer is related to the epidemiology of rubella. The virus is highly contagious and spreads rapidly among children, especially those in the preschool age group. Children in this age group are more likely to be exposed to the virus due to their increased social interactions and lack of immunity. This leads to a higher incidence of rubella in this age group.

**Why Each Wrong Option is Incorrect**
**Option A:** This age group is less likely to be affected due to the increased immunity in the community.

**Option B:** Although adults can contract rubella, it is not the most common age group affected.

**Option C:** This age group is more likely to be vaccinated and therefore less likely to be affected by rubella.

**Clinical Pearl / High-Yield Fact**
It is essential to note that rubella vaccination is a crucial component of measles, mumps, and rubella (MMR) vaccination, which is typically administered to children at 9-15 months and a booster dose at 4-6 years of age.

**Correct Answer:** C. Children in the preschool age group (1-4 years) are the most common age group affected by rubella.