**Core Concept**
The question is testing the knowledge of viral infections in pregnancy, specifically their impact on maternal and fetal mortality. Infections such as rubella, cytomegalovirus (CMV), and toxoplasmosis can cause significant morbidity and mortality in pregnant women and their offspring.

**Why the Correct Answer is Right**
Cytomegalovirus (CMV) is the most common cause of congenital viral infection and has the highest mortality rate among viral infections in pregnancy. CMV is a member of the herpesvirus family and can be transmitted to the fetus through vertical transmission, leading to congenital CMV infection. This infection can cause severe neurological damage, hearing loss, and vision impairment in the newborn.

**Why Each Wrong Option is Incorrect**
**Option A:** Rubella is a significant cause of congenital infection, but it is not the most common cause of mortality in pregnancy. While rubella infection during pregnancy can lead to congenital rubella syndrome, the mortality rate is lower compared to CMV.
**Option B:** Toxoplasmosis is another important cause of congenital infection, but it is not associated with the highest mortality rate in pregnancy. Toxoplasmosis infection during pregnancy can lead to congenital toxoplasmosis, but the severity of the disease is generally milder compared to CMV.
**Option D:** HIV is a significant cause of morbidity and mortality in pregnant women, but it is not a viral infection in the same sense as CMV, rubella, or toxoplasmosis. HIV infection during pregnancy can lead to vertical transmission and severe consequences for the newborn, but the question specifically asks about viral infections.

**Clinical Pearl / High-Yield Fact**
CMV is a common cause of congenital infection, and it is essential for healthcare providers to educate pregnant women about the risks of CMV transmission and the importance of good hygiene practices to prevent infection.

**Correct Answer:** C. Cytomegalovirus (CMV)