**Core Concept**
The question is testing knowledge of congenital infections, specifically viral infections that can be transmitted from mother to fetus during pregnancy. Congenital infections are a significant cause of morbidity and mortality in newborns.

**Why the Correct Answer is Right**
Toxoplasmosis is the most common congenital viral infection, caused by the protozoan parasite Toxoplasma gondii. It is estimated that 1 in 1000 live births are affected by congenital toxoplasmosis. The parasite can cross the placenta and infect the fetus, leading to serious complications such as hydrocephalus, chorioretinitis, and mental retardation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rubella is a viral infection, but it is not the most common congenital viral infection. While congenital rubella syndrome is a serious condition, it is much less common than congenital toxoplasmosis.
* **Option B:** Cytomegalovirus (CMV) is a common cause of congenital viral infection, but it is not the most common. CMV infection can also cause serious complications, including hearing loss and developmental delay.
* **Option C:** Herpes simplex virus (HSV) can cause congenital infection, but it is rare and typically occurs when the mother has active genital herpes lesions during delivery.

**Clinical Pearl / High-Yield Fact**
The most common congenital viral infection is toxoplasmosis, which can be prevented by avoiding undercooked meat and washing hands after handling cat litter.

**Correct Answer:** C.