**Core Concept**
Rubella, also known as German measles, is a highly contagious viral infection caused by the Rubella virus. The disease is characterized by a rash, fever, and lymphadenopathy, and its severity is highest in young children and non-immune pregnant women. The virus is typically most contagious before the onset of the rash.

**Why the Correct Answer is Right**
The correct answer is the age group most susceptible to Rubella infection. In children, the immune system is still developing, making them more prone to infections. Children under the age of 10 are more likely to contract Rubella due to their immature immune response. Additionally, children in this age group are more likely to be exposed to the virus in school settings, where the virus can easily spread.

**Why Each Wrong Option is Incorrect**
* **Option A:** This age group is not the most prone to Rubella infection, as the immune system is more developed in adolescents and young adults, making them less susceptible to the virus.
* **Option B:** While children in this age group may be susceptible to Rubella, they are not the most prone group compared to younger children.
* **Option C:** This age group is not typically associated with a high risk of Rubella infection, as the majority of cases occur in young children.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Rubella infection during pregnancy can lead to congenital Rubella syndrome, a condition that can cause serious birth defects. The risk of congenital Rubella syndrome is highest when the infection occurs during the first trimester of pregnancy.

**Correct Answer:** C. Children under the age of 10.